Semiconductor memory device capable of overcoming refresh disturb

ABSTRACT

Drains of first and second transistors are connected to a low level line of an internal circuitry such as a sense amplifier related to determination of a potential in a memory cell. The first transistor has its gate diode-connected to a sense drive line and its source grounded. The second transistor receives at its gate an internally generated signal, and its source is grounded. In the standby state, the potential of the sense drive line is set higher than low level of said word lines by the threshold voltage Vthn of the first transistor and used as dummy GND potential Vss′, and in the active state, the second transistor is rendered conductive so as to prevent floating of the sense drive line from the dummy GND potential Vss′.

This application is a continuation of application Ser. No. 10/163,468,filed Jun. 7, 2002, now abandoned which is a divisional of applicationSer. No. 09/860,795 (U.S. Pat. No. 6,414,883), filed, May 21, 2001,which is a divisional of application Ser. No. 09/168,962 file Oct. 9,1998 (U.S. Pat. No. 6,272,055), which is a continuation of applicationSer. No. 08/899,143, file Jul. 23, 1997, now U.S. Pat. No. 5,943,273,which is a continuation of 08/312,968, filed Sep. 30, 1994, now U.S.Pat. No. 5,687,123.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory device. Morespecifically, the present invention relates to a semiconductor memorydevice in which low level potential of sense amplifiers, memory cellsand bit lines constituting a memory cell array is adapted to be higherthan the low level of the word lines of the chip body, in a dynamicrandom access memory including a memory cell array arranged on asemiconductor substrate, sense amplifiers and circuitry for controllingthese.

2. Description of the Background Art

FIG. 101 is a schematic diagram showing a main portion of a conventionalDRAM. Referring to FIG. 101, a memory cell MC is connected to a wordline WL and a bit line pair BL, {overscore (BL)}. Bit line pair BL and{overscore (BL)} is connected to an n channel sense amplifier 2, anequalizer circuit 3 and a p channel sense amplifier 4 through transfergates Tr71 and Tr72. Transfer gates Tr71 and Tr72 are controlled by agate control signal BLI. To equalizer circuit 3, a VBL signal at thepotential of ½ Vcc as well as an EQ signal are applied. In response tothe EQ signal, equalizer circuit 3 precharges bit lines BL and{overscore (BL)} to ½ Vcc by VBL signal. Sense amplifiers 2 and 4 are toamplify a small potential difference read from the memory cell MC to thebit line pair BL and {overscore (BL)}. Sense amplifier 2 is activatedwhen a sense amplifier activating signal SO is applied to a sense driveline SN, while sense amplifier 4 is activated when an activating signal{overscore (SO)} is applied to a sense drive line SP.

FIG. 102 is a time chart showing the operation of the memory array shownin FIG. 101. There are a plurality of blocks of the memory array shownin FIG. 101, and each block is activated when a corresponding blockactivating signal is applied thereto. However, at this time, senseamplifiers 2 and 4 have not yet been activated. When data is to be readfrom memory cell MC, the BLI signal attains to the “H” level, transfergates TR71 and TR72 are rendered conductive, and bit line pair BL,{overscore (BL)} is connected to sense amplifiers 2 and 4 and toequalizer circuit 3. When word line WL rises to the boosted voltage Vppas shown in (a) of FIG. 102, a small potential difference is read frommemory cell MC to bit line pair BL and {overscore (BL)}, activatingsignal SO attains to the “H” level and activating signal {overscore(SO)} attains to the “L” level as shown in (b) and (c) of FIG. 102, andsense amplifiers 2 and 4 are activated, respectively. The smallpotential difference between the bit line pair BL and {overscore (BL)}is amplified by sense amplifiers 2 and 4, and the potential is enhancedto the level of “H” or “L”.

Now, the “L” level of the amplitude of the bit line pair BL and{overscore (BL)} is the low level of the word lines. In this case, the“L” level of a non-selected word line is equivalent to the “L” level ofthe amplitude of the bit line pair BL and {overscore (BL)}. Therefore,because of sub threshold leak current of the word line which is at thelow level of the word lines, charges stored in the memory cell MC flowsto the bit line and the amount of charges decrease, resulting inpossible destruction of the data in the memory cell MC. In order toprevent this phenomenon, conventionally, a negative voltage bias Vbb isapplied to the memory array portion. However, it requires a negativepotential generating circuit for generating the negative voltage biasVbb. In addition, this approach has disadvantage such as increase ofarray noise as the current incidental to memory array operation flows tothe side of the ground, floating of the “L” level of the non-selectedword line, increase of the sub threshold leak current of the word lineand degradation of the refresh characteristics.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide asemiconductor memory device in which threshold voltage of memory celltransistors can be set low and reliability can be improved, and inaddition, which eliminates the need of a triple well structure.

Briefly stated, the semiconductor memory device of the present inventionincludes a memory cell array including memory cells each connected toone of a plurality of bit lines and one of a plurality of word lines; asense amplifier for amplifying a small potential difference read fromthe memory cell array to the bit line, a control circuit for controllingreading of data from the memory cell array and writing of data to thememory cell array, and a potential setting circuit for setting lines oflow level potential in the sense amplifier, the memory cells and bitlines to a potential higher than the low level of the word lines.

Therefore, according to the present invention, since the lines of thelow level potential of the sense amplifier group, the memory cells andthe bit lines are set to a potential higher than the low level of theword lines, the threshold voltage of the memory cell transistor can beset lower, reliability can be improved, a boosted voltage generatingcircuit becomes unnecessary, and the triple well structure becomesunnecessary.

More preferably, in order to enhance the potential of the low levelpotential line by the threshold voltage of a semiconductor element, thepotential setting circuit discharges the potential of the low levelpotential line by a second semiconductor element in response to a signalwhich corresponds to a period in which large current flows.

More preferably, the potential setting circuit includes a referencevoltage generating circuit for generating a reference voltage which isapproximately equal to the low level potential, and a potentialcompensating circuit for comparing the reference voltage with the lowlevel potential line, and for compensating the potential of the lowlevel potential line so that the potential becomes higher than the lowlevel of the word lines. The potential compensating circuit includes acomparing circuit and a switching circuit which switches in response tothe comparison output from the comparing circuit.

Further, potential setting circuit includes a sustain circuit forintermittently supplying a power supply potential to the low levelpotential line for compensating the potential thereof so that it attainsa level higher than the low level of the word lines. The sustain circuitincludes an oscillating circuit and a pumping circuit.

More preferably, the potential setting circuit includes a referencevoltage generating circuit for generating a reference potential, acomparing circuit for comparing the reference voltage with the potentialof the low level potential line, and a switching circuit for dischargingthe potential of the low level potential line to the low level of theword lines side in accordance with the output from the comparingcircuit.

More preferably, a low level lowering preventing circuit such as a diodeis provided for preventing lowering of the potential of the low levelpotential line from the potential higher than the low level of the wordlines.

More preferably, a voltage comparison stopping circuit for disabling thevoltage comparing circuit while a large current flows, and floatingpreventing circuit for preventing floating of the potential of the lowlevel potential line by forcefully operating the switching circuit whilethe large current flows are provided.

Further, more preferably, the sense amplifier includes a switchingelement connected between the low level potential line and the groundfor enhancing the potential of the low level potential line by thethreshold voltage thereof. The switching element includes a switchingcircuit which is rendered conductive when an input potential becomesequal to or lower than the low level of the word lines for applying anegative potential to an input electrode of the switching element whilea large current flows so as to make short the response time. Theswitching circuit applies the low level of the word lines to the inputelectrode of the switching element in the former half and a negativepotential in the latter half of the period in which the large currentflows.

According to the another aspect, the present invention provides asemiconductor memory device having an internal circuit to which a powersupply voltage is applied externally, which includes a potential settingcircuit for setting a high level potential supplied in the internalcircuit to a potential different from the externally supplied powersupply voltage, and for setting a low level potential supplied in theinternal circuit different from the low level of the word lines, and acircuit for changing the high level and low level potentials dependenton whether the semiconductor memory device is in and not in operation.

In accordance with another aspect, the present invention provides asemiconductor memory device in which a chip is formed on a semiconductorsubstrate, which includes a memory cell array including memory cellseach connected to one of a plurality of bit lines and one of a pluralityof word lines; a sense amplifier for amplifying a small potentialdifference read from the memory cell array to the bit line; a controlcircuit for controlling reading of data from the memory cell array andwriting of data to the memory cell array; a substrate potentialgenerating circuit for supplying a negative level substrate potential tothe semiconductor substrate; a boosted voltage generating circuit forgenerating a boosted voltage to be supplied to the word line; and apotential setting circuit for switching the boosted voltage potentialand the negative potential to arbitrary potentials dependent on whetherthe chip is in use or not in use.

In accordance with a still another aspect of the present invention, thesemiconductor memory device includes a memory cell array including aplurality of memory cells each connected to one of a plurality of bitlines and one of a plurality of word lines, a sense amplifier foramplifying a small potential difference read from the memory cell arrayto a bit line, a control circuit for controlling reading of data fromthe memory cell array and writing of data to the memory cell array, apotential setting circuit for setting a low level potential line of thebit lines, memory cells and sense amplifier to a potential higher thanthe low level of the word lines, and a potential compensating circuitfor compensating the set low level potential.

According to a still another aspect of the present invention, thesemiconductor memory device includes a memory cell array including aplurality of memory cells each connected to one of a plurality of bitlines and one of a plurality of word lines, a sense amplifier foramplifying a small potential difference read from the memory cell arrayto a bit line, a control circuit for controlling reading of data fromthe memory cell array and writing of data to the memory cell array, apotential setting circuit for setting a low level potential line of thebit lines, memory cells and sense amplifier to a potential higher thanthe low level of the word lines, a potential elevating compensatingcircuit responsive to lowering of the set potential higher than the lowlevel of the word lines, compensating for the lowering by elevating thepotential, and voltage lowering compensating circuit responsive to riseof the potential for compensating the rise by lowering the potential.

According to a still further aspect of the present invention, thesemiconductor memory device includes a memory cell array includingmemory cells each connected to one of a plurality of bit lines and oneof a plurality of word lines, a sense amplifier for amplifying a smallpotential difference read from the memory cell array to the bit line, adriving line for driving the sense amplifier, and a potential settingmeans for setting, when the sense amplifier is driven, the low levelpotential of the driving line to a potential higher than the low levelof the word lines.

According to a still further aspect of the present invention, thesemiconductor memory device includes a memory cell array includingmemory cells each connected to one of a plurality of bit lines and one aplurality of word lines, a sense amplifier for amplifying a smallpotential difference read from the memory cell array to a bit line, atransfer gate connected between the bit line and the sense amplifier,and a control circuit for setting, when the sense amplifier is driven,the gate potential of the transfer gate to the low level of the wordlines and the low level potential of the bit line to the thresholdvoltage of the transfer gate.

According to a still further aspect of the present invention, thesemiconductor memory device includes a memory cell array including aplurality of memory cells each connected to one of a plurality of bitlines and one of a plurality of word lines, a sense amplifier foramplifying a small potential difference read from the memory cell arrayto a bit line, a control circuit for controlling reading of data fromthe memory cell array and writing of data to the memory cell array, anda potential setting circuit for setting low level potential line of thebit lines, memory cells and sense amplifier to a potential higher thanthe low level of the word lines, and for setting a high level potentialto a potential lower than the power supply voltage level externallyapplied.

According to a still further aspect of the present invention, asemiconductor memory device provided with chips formed on asemiconductor substrate includes a memory cell array including memorycells each connected to one of a plurality of bit lines and one of aplurality of word lines, a sense amplifier for amplifying a smallpotential difference read from the memory cell array to a bit line, acontrol circuit for controlling reading of data from the memory cellarray and writing of data to the memory cell array, a potential settingcircuit for setting low level potential line of bit lines, memory cellsand sense amplifier to a potential higher than the low level of the wordlines, and a low level of the word lines forcing circuit for forcing,when data retention time of a memory cell is to be tested, the low levelpotential line of the bit lines, memory cell and sense amplifier to thelow level of the word lines.

According to a still further aspect of the present invention, thesemiconductor memory device provided with chips formed on asemiconductor substrate includes a memory cell array including memorycells each connected to one of a plurality of bit lines and one of aplurality of word lines, a word line driving circuit for driving a wordline, a sense amplifier for amplifying a small potential difference readfrom the memory cell array to a bit line, a control circuit forcontrolling reading of data from the memory cell array and writing ofdata to the memory cell array, and a potential setting circuit forsetting, when data retention time of the memory cell is to be tested,the low level potential line of the word line driving circuit to apotential higher than the low level of the word lines.

According to a still further aspect, the semiconductor memory deviceprovided with chips formed on the semiconductor substrate includes amemory cell array including memory cells each connected to one of aplurality of bit lines and one of a plurality of word lines, a senseamplifier for amplifying a small potential difference read from thememory cell array to a bit line, a control circuit for controllingreading of data from the memory cell array and writing of data to thememory cell array, a substrate potential generating circuit forsupplying a negative level substrate potential to the semiconductorsubstrate, and a substrate potential setting circuit for setting, whendata retention time of the memory cell is to be tested, the substratepotential of the semiconductor substrate to a potential higher than thenegative level substrate potential.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are illustrations comparing concepts of the prior artand of the present invention.

FIG. 2 is a schematic diagram showing a first embodiment of a firstaspect of the present invention.

FIG. 3 is a time chart showing the operation of the circuit of FIG. 2.

FIG. 4 is a schematic diagram showing a second embodiment in accordancewith the first aspect of the present invention.

FIG. 5 is a schematic diagram showing a first embodiment in accordancewith a second aspect of the present invention.

FIG. 6 is a schematic diagram showing a second embodiment in accordancewith the second aspect of the present invention.

FIG. 7 is a schematic diagram showing a third embodiment in accordancewith the second aspect of the present invention.

FIG. 8 is a schematic diagram showing a fourth embodiment in accordancewith the second aspect of the present invention.

FIG. 9 is a time chart showing the operation of the circuit shown inFIG. 8.

FIG. 10 is a schematic diagram showing a first embodiment in accordancewith a third aspect of the present invention.

FIG. 11 is a time chart showing the operation of the embodiment shown inFIG. 10.

FIG. 12 is a schematic diagram showing a second embodiment in accordancewith the third aspect of the present invention.

FIG. 13 is a time chart showing the operation of the embodiment of FIG.12.

FIG. 14 is a time chart showing operation of a third embodiment inaccordance with the third aspect of the present invention.

FIG. 15 is a schematic diagram showing a first embodiment in accordancewith a fourth aspect of the present invention.

FIG. 16 shows an example of a switch circuit shown in FIG. 15.

FIG. 17 is a time chart showing operation of the embodiment of FIG. 15.

FIG. 18 is a block diagram showing a second embodiment in accordancewith the fourth aspect of the present invention.

FIG. 19 is a schematic diagram showing an example of a switch circuitshown in FIG. 18.

FIG. 20 is a time chart showing the operation of the embodiment shown inFIG. 18.

FIG. 21 is a time chart showing an operation of a third embodiment inaccordance with the fourth aspect of the present invention.

FIG. 22 is a schematic diagram showing a first embodiment in accordancewith a fifth aspect of the present invention.

FIG. 23 is a schematic diagram showing a first embodiment in accordancewith a sixth aspect of the present invention.

FIG. 24 is a time chart showing the operation of the embodiment of FIG.23.

FIG. 25 is a schematic diagram showing a second embodiment in accordancewith the sixth aspect of the present invention.

FIG. 26 is a schematic diagram showing a third embodiment in accordancewith the sixth aspect of the present invention.

FIG. 27 is a time chart showing the operation of the embodiment of FIG.26.

FIG. 28 is a schematic diagram showing a fourth embodiment in accordancewith the sixth aspect of the present invention.

FIG. 29 is a schematic diagram showing a fifth embodiment in accordancewith the sixth aspect of the present invention.

FIG. 30 is a schematic diagram showing a sixth embodiment in accordancewith the sixth aspect of the present invention.

FIG. 31 is a schematic diagram showing a seventh embodiment inaccordance with the sixth aspect of the present invention.

FIG. 32 is a schematic diagram showing an eighth embodiment inaccordance with the sixth aspect of the present invention.

FIG. 33 is a schematic diagram showing an embodiment in which a currentsensor of FIG. 32 is used as a comparing circuit.

FIG. 34 is a schematic diagram showing another example of the currentsensor.

FIG. 35 is a schematic diagram showing a further example of the currentsensor.

FIG. 36 is a schematic diagram of a current sensor provided withhysteresis characteristic.

FIG. 37 is a block diagram showing an example in which the presentinvention in accordance with the sixth aspect is applied to a testcircuit for testing data retention time of a memory cell.

FIG. 38 shows an example in which the present invention in accordancewith the sixth aspect is applied to each memory block.

FIG. 39 shows another example in which the present invention inaccordance with the sixth aspect is applied to each memory block.

FIG. 40 shows a further example in which the present invention inaccordance with the sixth aspect is applied to each memory block.

FIG. 41 shows a still further embodiment in which the present inventionin accordance with the sixth aspect is applied to each memory block.

FIG. 42 is a schematic diagram showing a first embodiment in accordancewith a seventh aspect of the present invention.

FIG. 43 is a time chart showing the operation of the embodiment shown inFIG. 42.

FIG. 44 is an illustration of path through which a sub threshold currentof the memory cell flows.

FIG. 45 is a cross section of the first embodiment in accordance withthe seventh aspect of the present invention.

FIG. 46 shows a circuit for generating row predecoder outputs Xi, j, k.

FIG. 47 shows a circuit for generating a master row decode signal φxi.

FIG. 48 is a schematic diagram showing a word line driving signalgenerating circuit.

FIG. 49 shows a column SFU/L signal generating circuit.

FIG. 50 shows a circuit for generating a CSL signal.

FIG. 51 is a schematic diagram showing a second embodiment in accordancewith the seventh aspect of the present invention.

FIG. 52 is a time chart showing the operation of the embodiment shown inFIG. 51.

FIG. 53 is a schematic diagram showing a third embodiment in accordancewith the seventh aspect of the present invention.

FIG. 54 is a time chart showing the operation of the embodiment shown inFIG. 53.

FIG. 55 is a schematic diagram showing a fourth embodiment in accordancewith the seventh aspect of the present invention.

FIG. 56 is a time chart showing the operation of the embodiment shown inFIG. 55.

FIG. 57 is an illustration of the principle of an eighth aspect of thepresent invention.

FIG. 58 is an illustration showing sub threshold leak current of theword line in the present invention in accordance with the eighth aspect.

FIG. 59 is a schematic diagram showing a first embodiment in accordancewith the eighth aspect of the present invention.

FIG. 60 shows an IC operation voltage in the embodiment shown in FIG.59.

FIG. 61 shows a change in a reference voltage in the embodiment shown inFIG. 59.

FIG. 62 is an electric circuit diagram showing an example of a referencevoltage generating circuit for generating the reference voltage detectedin FIG. 59.

FIG. 63 shows an improvement of the reference voltage generating circuitshown in FIG. 62.

FIG. 64 shows a further improvement of the reference voltage generatingcircuit shown in FIG. 62.

FIG. 65 is a diagram of waveforms showing the operation of the referencevoltage generating circuit of FIG. 64.

FIG. 66 shows a further example of the reference voltage generatingcircuit.

FIG. 67 shows a still further example of the reference voltagegenerating circuit.

FIG. 68 is a schematic diagram of an embodiment in which powerconsumption is reduced in refreshing operation, when the chip is notused.

FIG. 69 is a time chart showing the operation of the embodiment of FIG.68.

FIG. 70 is a time chart showing the operation of the embodiment of FIG.68.

FIG. 71 is an illustration of an embodiment in which operations of asubstrate potential generating circuit and of a boosted voltagegenerating circuit are controlled when the chip is not used.

FIG. 72 shows an example of a substrate potential generating circuit.

FIG. 73 shows an example of the boosted voltage generating circuit.

FIG. 74 is a schematic diagram in a LSI when the invention in accordancewith the eighth aspect is applied.

FIG. 75 shows a first embodiment in accordance with a ninth aspect ofthe present invention.

FIG. 76 is a schematic diagram of a circuit for generating a referencevoltage Vref and a gate voltage Vp shown in FIG. 75.

FIG. 77 shows an example in which a resistor shown in FIG. 76 isreplaced by a transistor.

FIG. 78 shows an improvement of the embodiment shown in FIG. 75.

FIG. 79 shows another embodiment in accordance with the ninth aspect ofthe present invention.

FIG. 80 shows an improvement of the embodiment shown in FIG. 78.

FIG. 81 shows an improvement of the example shown in FIG. 80.

FIG. 82 is a schematic block diagram showing a first embodiment inaccordance with a tenth aspect of the present invention.

FIG. 83 is a time chart showing the operation of the embodiment shown inFIG. 82.

FIG. 84 is a block diagram showing a second embodiment in accordancewith the tenth aspect of the present invention.

FIG. 85 is a schematic diagram showing a dummy GND level generatingcircuit in the embodiment shown in FIG. 84.

FIG. 86 is a schematic block diagram showing a third embodiment inaccordance with the tenth aspect of the present invention.

FIG. 87 is a schematic block diagram showing a fourth embodiment inaccordance with the tenth aspect of the present invention.

FIG. 88 is a time chart showing the operation of the embodiment of FIG.87.

FIG. 89 is a schematic block diagram showing a fifth embodiment inaccordance with the tenth aspect of the present invention.

FIG. 90 is a schematic diagram showing a switch of the embodiment shownin FIG. 89.

FIG. 91 is a schematic block diagram showing a sixth embodiment inaccordance with the tenth aspect of the present invention.

FIG. 92 shows, in enlargement, a main portion of the embodiment shown inFIG. 91.

FIG. 93 is a block diagram showing a circuit providing a sense amplifieractivating signal S0N1.

FIG. 94 is a schematic block diagram showing a circuit providing a senseamplifier activating signal S0N2.

FIG. 95 shows an improvement of the example shown in FIG. 92.

FIG. 96 shows, in enlargement, a main portion of a seventh embodiment inaccordance with the tenth aspect of the present invention.

FIG. 97 shows an improvement of the example shown in FIG. 96.

FIG. 98 is a schematic block diagram showing an eighth embodiment inaccordance with the tenth aspect of the present invention.

FIG. 99 is a time chart showing the operation of the embodiment shown inFIG. 98.

FIG. 100 is a schematic block diagram showing a ninth embodiment inaccordance with the tenth aspect of the present invention.

FIG. 101 is a schematic diagram of a conventional semiconductor memorycell array.

FIG. 102 is a time chart showing the operation of the memory cell arrayshown in FIG. 101.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A and 1B are illustrations for comparing the concepts of theprior art and of the present invention. In the prior art example,referring to FIG. 1A, an internal power supply voltage Int. Vcc (3.3V)is generated from an external power supply voltage Ext.Vcc (5V), apotential between the internal power supply voltage Int. Vcc and the lowlevel of the word lines GND (0V) is being applied to the memory cell,and it is necessary to apply a negative potential Vbb (−2V) to thememory cell in order to suppress the sub threshold leak. For thisreason, a negative potential generating circuit was necessary in theprior art.

By contrast, in the present invention, referring to FIG. 1B, the “L”level of the amplitude of the bit line is set not to the low level ofthe word lines GND but to a dummy GND level (VGND) (0.5V) which is newlygenerated between the bit line precharge level and the low level of theword lines GND. In this case, the “L” level (GND) of the non-selectedword line is in a state relatively biased negative with respect to the“L” level of the bit line amplitude.

FIG. 2 is a schematic diagram showing a first embodiment in accordancewith the first aspect of the present invention. Referring to FIG. 2, aninternal circuit 5 is related to determination of memory cell potential,such as a bit line charging/discharging circuit (sense amplifiercircuit), a half Vcc generating circuit or the like, and not the wholecircuitry of the chip (it should be noted that the word line drivingcircuit is not included). In the prior art, internal circuit 5 isconnected to the low level of the word lines. However, in thisembodiment, it is connected to a dummy GND line 30. To dummy GND line30, the gate and the drain of an n channel transistor Tr1 as well as thedrain of an n channel transistor Tr2 are connected. The n channeltransistors Tr1 and Tr2 have their sources connected to the low level ofthe word lines Vss. An internally generated signal φ is applied to thegate of n channel transistor Tr2. The n channel transistor Tr1 elevatesthe potential Vss′ of the dummy GND line 30 by its threshold voltageVthn. However, when a large current flows from the memory cell array tothe ground when active, especially at the time of charging/dischargingthe bit lines or at the time when the bit line is connected to the I/Oline because of the change in the column address, it is difficult tomaintain the level Vss′ of the dummy GND line 30 near the thresholdvoltage Vthn only by means of the n channel transistor Tr1, in view ofdrivability. The reason for this is that n channel transistor Tr1operates near the threshold voltage and has relatively high resistance,with the level Vss′ of the dummy GND line 30 being near the thresholdvoltage Vthn. Accordingly, only at the time of charging/discharging thebit lines or at the time when the bit line is connected to the I/O linebecause of the change in the column address, n channel transistor Tr2 isrendered conductive by the internally generated signal φ, so as tosuppress floating of the potential Vss′ of the dummy GND line 30.

FIG. 3 is a time chart showing the operation of the example of FIG. 2.The operation of the example of FIG. 2 will be described with referenceto FIG. 3. As shown in (a) of FIG. 3, in the standby state in which arow address strobe signal {overscore (RAS)} is at the “H” level, theinternally generated signal φ is at the “L” level as shown in FIG. 3(f).After the row address strobe signal {overscore (RAS)} falls to the “L”level, a word line is activated by the row address shown in (c) of FIG.3, information in the memory cell is transmitted to the bit line, asense amplifier activating signal {overscore (SE)} attains to the “L”level as shown in (d) of FIG. 3, and the sense amplifier is activated.At this time, the total capacitance of the bit lines connected to theactivated word line is charged/discharged. In response to the senseamplifier activating signal {overscore (SE)}, the internally generatedsignal φ rises to the power supply voltage Vcc, and after the lapse of aprescribed time period t1, falls to the low level of the word lines Vss.Consequently, in a time period t1, the resistance between the low levelof the word lines Vss and the potential Vss′ of the dummy GND line isreduced, enabling sensing operation at high speed. In the period t1,though the potential Vss′ of the dummy GND line is drawn near to the lowlevel of the word lines Vss, it floats by some extent as shown in (g) ofFIG. 3 because of the resistance component of n channel transistor Tr2,since a large current flows for charging/discharging the bit lines. Bysetting the time t1 such that the amount of floating attainsapproximately the same level as the threshold voltage Vthn in thisperiod, floating of Vss′ caused by n channel transistor Tr2 can beprevented. More specifically, if n channel transistor Tr2 is notconnected to dummy GND line 30, the potential Vss′ of dummy GND line 30floats as shown by the dotted line in (g) of FIG. 3. However, because ofthe function of n channel transistor Tr2, floating can be suppressed asshown by the solid line of (g) of FIG. 3.

After the sensing operation, in a read cycle, a column activating signal{overscore (CE)} shown in (e) of FIG. 3 is activated by the change inthe column address, a column selection line is activated and a specificbit line is connected to an I/O line. The potential of the I/O lineflows in to the bit line, and the potential Vss′ of dummy GND line 30floats as shown in (g) of FIG. 3. In this case also, in response toactivation of the column activating signal {overscore (CE)}, theinternally generated signal φ is set to and kept at the level of the lowlevel of the word lines of a time period t2, as in the case of senseactivation. In the write cycle also, the same effect can be obtained bysetting the internally generated signal φ to the level of the low levelof the word lines Vcc for a period of time corresponding to the timingof writing data from the I/O line to the bit line. Here, as the timingof the internally generated signal φ, sense amplifier activating signalSE and column activating signal {overscore (CE)} are described asexamples in this embodiment. However, any internal signal generatedaround the timing of the start of sensing operation or connection of I/Oline and the bit line may be used to generate the signal φ. The nchannel transistor Tr has its gate and drain short-circuited andconnected to the dummy GND line 30 and its source connected to the lowlevel of the word lines Vss. However, a p channel transistor having itsgate and drain short-circuited and connected to the low level of theword lines Vss and its source connected to the dummy GND line 30 may beused. Though the internally generated signal φ is activated at a timingaround the start of operation of the sense amplifier and at a timingaround the connection of the I/O line and the bit line in the aboveembodiment, the signal may be activated at either of these timings.

FIG. 4 is a schematic diagram showing a second embodiment in accordancewith the first aspect of the present invention. In this embodiment shownin FIG. 4, a clamp circuit 7 is connected to the dummy GND line 30 inorder to prevent lowering of the level of the dummy GND line 30 causedby the operation of n channel transistor Tr2 in the embodiment shown inFIG. 3. Clamp circuit 7 includes a differential amplifying circuit 71comparing the potential Vss′ of the dummy GND line 30 with a referencevoltage, and an n channel transistor Tr3 receiving the output fromdifferential amplifying circuit 71 for charging the potential of dummyGND line 30. The n channel transistor Tr3 has its gate connected to theoutput of differential amplifying circuit 71, its drain connected to thepower supply potential Vcc and its source connected to the dummy GNDline 30. When the level Vss′ of the dummy GND line 30 is lower than thereference voltage, differential amplifying circuit 71 provides a “H”level signal so as to render n channel transistor Tr3 conductive, andprovides a “H” level signal from the power supply potential Vcc to dummyGND line 30 to charge the same. If the potential of dummy GND line 30rises, differential amplifying circuit 71 renders n channel transistorTr3 non-conductive, and suppresses lowering of the level of dummy GNDline 30 caused by the mismatch of the conduction period t1 or t2 of nchannel transistor Tr2, which has been described with reference to FIG.3 above.

FIG. 5 shows a first embodiment in accordance with a second aspect ofthe present invention. The embodiment shown in FIG. 5 includes areference voltage generating circuit 81 generating a voltage ofapproximately the same level as that of dummy GND line 30, adifferential amplifying circuit 8 for comparing the level of dummy GNDline 30 with the reference voltage, and an n channel transistor Tr3receiving the output from differential amplifying circuit 8. The nchannel transistor Tr3 has its gate connected to an output ofdifferential amplifying circuit 8, its drain connected to dummy GND line30 and its source connected to the low level of the word lines Vss. Inthis embodiment, if the level of dummy GND line 30 is higher than thereference voltage output from reference voltage generating circuit 81, a“H” level signal is applied from the differential amplifying circuit 8to the gate of n channel transistor Tr3. In response, n channeltransistor Tr3 is rendered conductive, discharging the potential ofdummy GND line 30. When the potential of dummy GND line 30 becomes lowerthan the reference voltage, differential amplifying circuit 8 stopsdischarging by n channel transistor Tr3, so that the level of the dummyGND line 30 is maintained at a level Vss′ which is higher than the lowlevel of the word lines.

FIG. 6 is a schematic diagram showing a second embodiment in accordancewith a second aspect of the present invention. The embodiment of FIG. 6is an improvement of the embodiment shown in FIG. 5, preventing loweringof the level of dummy GND line 30 caused by response delay ofdifferential amplifying circuit 8. More specifically, a diode D1 isconnected between the drain of n channel transistor Tr3 and the dummyGND line 30. Since diode G1 is connected between the drain of n channeltransistor Tr3 and dummy GND line 30, when a small current is flowingthrough diode D1, dummy GND line 30 is not pulled to the level of thelow level of the word lines Vss but elevated by the diffusion potential(of about 0.6V) of the diode D1, and therefore lowering of the level ofdummy GND line 30 can be suppressed and stable potential of dummy GNDline 30 can be obtained.

FIG. 7 is a schematic diagram showing a third embodiment in accordancewith a second aspect of the present invention. The embodiment shown inFIG. 7 is an improvement of the embodiment shown in FIG. 6 and loweringof the level of dummy GND line 30 caused by response delay ofdifferential amplifying circuit 8 is suppressed. More specifically, adecoupling capacitor C1 is connected between the low level of the wordlines and a node between diode D1 and the drain of n channel transistorTr3. Generally, a decoupling capacitor should be connected to dummy GNDline 30. However, in this example, current change caused by theoperation of internal circuit 5 is absorbed by the decoupling capacitor,hindering the compensating effect of diode D1. Accordingly, a decouplingcapacitor C1 is connected between the ground and a node between diode D1and a drain of n channel transistor Tr3 so that the compensation at thediffusion potential by diode D1 is ensured, and lowering of the level ofdummy GND line 30 can be suppressed, providing stable potential Vss′.

FIG. 8 is a schematic diagram showing a fourth embodiment in accordancewith a second aspect of the present invention. This embodiment is animprovement of the embodiment shown in FIG. 5, in which floating of thelevel of dummy GND line 30 caused by response delay of differentialamplifying circuit 8 when a large current is consumed is improved. Thisembodiment is a combination of the embodiment shown in FIG. 5 andembodiment of FIG. 2. More specifically, a differential amplifyingcircuit stopping circuit consisting of an n channel transistor Tr4 isconnected to differential amplifying circuit 8, and a float preventingcircuit consisting of an n channel transistor Tr5 is connected to thegate of n channel transistor Tr3. More specifically, n channeltransistor Tr4 has its drain connected to an activation input terminalof differential amplifying circuit 8, and its source grounded. The nchannel transistor Tr5 has its drain connected to the power supplypotential Vcc, and its source connected to the gate of n channeltransistor Tr3. Internally generated signal φ is inverted by an inverter29 and applied to the gates of n channel transistors Tr4 and Tr5.

FIG. 9 is a time chart showing the operation of the embodiment of FIG.8. The operation of the embodiment shown in FIG. 8 will be describedwith reference to FIG. 9. At a timing when large current flows in thememory cell array, that is, when sensing operation is started or whenthe I/O line is connected to the bit line, upon reception of senseamplifier activating signal {overscore (SE)} shown in (d) of FIG. 9 orcolumn activating signal {overscore (CE)} shown in (e) of FIG. 9, aninternally generated signal φ shown in (f) of FIG. 9 is inverted by ainverter 9, and n channel transistor Tr4 is rendered conductive, so thatoperation of differential amplifying circuit 8 is stopped. Wheninternally generated signal φ attains to the “L” level, n channeltransistor Tr5 is rendered conductive, a “H” level signal is forcefullyapplied to the gate of n channel transistor Tr3 from power supplypotential Vcc, forcing the operation of n channel transistor Tr3, sothat floating of dummy GND line 30 is prevented.

FIG. 10 is a schematic diagram showing a first embodiment in accordancewith a third aspect of the present invention. The embodiment of FIG. 10is adapted such that when a sense amplifier drive line SN of senseamplifier 2 is at “L” level, the level of the dummy GND line 30 is setto a potential Vss′ which is higher than the ground potential Vss.

Generally, in the reading operation of a DRAM, after the fall of a rowaddress strobe signal {overscore (RAS)}, a word line WL is activated bya row address, information in the memory cell is transmitted to the bitline BL and amplified by sense amplifier 2, as already described withreference to FIG. 101. Sense amplifiers 2 and 4 are basically formed byn and p channel two sense amplifiers as shown in FIG. 101 and in each ofthe sense amplifiers 2 and 4, gates and drains of two transistors arecross coupled and the sources are connected together, and this commonlyconnected node is connected to the ½ (Vcc÷Vs) line through an n channeltransistor Tr6. When n channel transistor Tr6 is rendered conductive,sensing operation starts, the small potential difference of the bit lineBL is activated to the level of the power supply line, and re-writing isperformed to the memory cell. In this embodiment shown in FIG. 10, a pchannel transistor Tr7 is connected between sense amplifier drive lineSN and the low level of the word lines Vss. The potential of senseamplifier drive line SN is adapted not to be lower than the thresholdVthp of p channel transistor Tr7. The internally generated signal φ isapplied to the gate of p channel transistor Tr7.

FIG. 11 is a time chart showing the operation of FIG. 10. Referring toFIG. 11, the operation of the embodiment shown in FIG. 10 will bedescribed. At the standby state, referring to (a) of FIG. 11, rowaddress strobe signal {overscore (RAS)} is at the “H” level, andtherefore, internally generated signal φP is at the “H” level, as shownin (d) of FIG. 11. Since internally generated signal φp is applied tothe gate of n channel transistor Tr6, n channel transistor Tr6 isrendered conductive, and sense amplifier drive line SN is precharged tothe level of ½ (Vcc÷Vthp). When row address strobe signal {overscore(RAS)} falls to “L” and sense amplifier activating signal {overscore(SE)} falls to the “L” level as shown in (b) of FIG. 11, internallygenerated signal φ attains to the “L” level. In response to theinternally generated signal φ, p channel transistor Tr7 is renderedconductive, and sensing operation starts. When the level of senseamplifier drive line SN attain to approximately the same level as thethreshold voltage Vthp of p channel transistor Tr7, it is renderednon-conductive, the final level of the sense amplifier 2 on the side of“L” level reaches the threshold voltage Vthp, and the “L” level writtento the memory cell is equivalent to the threshold voltage Vthp.

FIG. 12 is a schematic diagram showing a second embodiment in accordancewith a third aspect of the present invention. The embodiment of FIG. 12is an improvement of the embodiment shown in FIG. 10. In thisembodiment, improvement is made to prevent floating of the voltage ofsense amplifier drive line SN caused by p channel transistor Tr7operating near the threshold voltage and as a result having higherresistance, when large current is consumed. More specifically,internally generated signal φ applied to the gate of p channeltransistor Tr7 is generated from a switching circuit shown in FIG. 12.The switching circuit includes n channel transistors Tr8 and Tr9. To thegate of n channel transistor Tr8, a control signal {overscore (φt )} isapplied which signal is at the “H” level only for a period t1 from thefall of sense amplifier activating signal {overscore (SE)} to the “L”level and at the “L” level except that period. To the gate of n channeltransistor Tr9, a control signal φt is applied which signal is at the“L” level only for the period t1 and except this period at “H” level.The n channel transistor Tr8 has its source grounded, and n channeltransistor Tr9 receives at its drain a negative voltage −Vb fromnegative potential generating circuit 9. In order to obtain muchimprovement, the negative voltage −Vb should preferably be selected tobe larger than the threshold voltage Vthp of p channel transistor Tr7 ofFIG. 10. The drain of n channel transistor 8 and the source of n channeltransistor Tr9 are commonly connected to the gate of p channeltransistor Tr7 of FIG. 10.

FIG. 13 is a time chart showing the operation of the circuit of FIG. 12.Referring to FIG. 13, the operation of the embodiment shown in FIG. 12will be described. When sense amplifier activating signal {overscore(SE)} falls to the “L” level as shown in (b) of FIG. 13, control signalφt rises to “H” level, and n channel transistor Tr9 is renderedconductive. Consequently, the negative voltage −Vb is applied to thegate of p channel transistor Tr7 of FIG. 10 as the internally generatedsignal φ. This facilitates conduction of p channel transistor Tr7,reduces resistance, and therefore floating of sense amplifier drive lineSN can be prevented. More specifically, when an internally generatedsignal φ of which “L” level is the low level of the word lines Vss isapplied as shown in (c) of FIG. 11 to the gate of p channel transistorTr7 shown in FIG. 10, the wave of sense amplifier drive line SN fallingfrom “H” level to “L” level becomes moderate as shown by the dotted lineof (d) of FIG. 13, and accordingly, the potential of bit lines BL,{overscore (BL)} fall moderately as shown by the dotted line of (e) ofFIG. 13, hindering high speed operation. By contrast, as the negativepotential −Vb is applied to the gate of p channel transistor Tr7 onlyfor the period t1 at the start of sensing at which period a largecurrent flows, by means of the switch circuit of the embodiment shown inFIG. 12, the sense amplifier drive line SN can be steeply lowered asshown in (d) of FIG. 13, realizing higher speed of operation. After thelapse of time t1, n channel transistor Tr8 is rendered conductive bycontrol signal {overscore (φt)}, and internally generated signal φattains to the level of the low level of the word lines Vss.

FIG. 14 is a time chart showing a third embodiment in accordance with athird aspect of the present invention. This embodiment is a furtherimprovement of FIG. 13, in which floating of sense amplifier drive lineSN caused by high resistance of p channel transistor Tr7 of FIG. 10 isfurther suppressed. More specifically, of the period t1 at the start ofsensing operation when a large current flows, the internally generatedsignal φ is kept at the level of the low level of the word lines Vss ofthe former half period t2, and kept at the negative potential −Vb levelonly for the latter half period t3. Therefore, to the gate of n channeltransistor Tr8 shown in FIG. 12, a control signal {overscore (φt)} whichis kept at the “H” level only the period t2 and after the period t1 isapplied. In this embodiment, since the period in which the negativepotential −Vb is applied to the gate of p channel transistor Tr7 can bemade shorter than in the embodiment of FIG. 12, load of the negativepotential generating circuit 9 can be reduced.

FIG. 15 is a schematic diagram showing a first embodiment in accordancewith a fourth aspect of the present invention. In the embodiment shownin FIG. 15, a circuit generating a dummy GND level signal is provided inthe semiconductor substrate and the sense amplifier amplifies the bitline potential between the power supply potential Vcc and the dummy GNDline 30, wherein the ground level of a word drive circuit driving theword line is adapted to be switched between the low level of the wordlines Vss and the level Vss′ of the dummy GND line 30 by means ofswitching circuit 12. More specifically, row decoder 11 decodes a rowaddress signal and applies the decoded word signal to a word driver 10.Word driver 10 includes a p channel transistor 71 and an n channeltransistor 72, and to the gates of p channel transistor 71 and n channeltransistor 72, the word signal is applied from row decoder 11. To thedrain of p channel transistor 71, a boosted voltage Vpp is applied, andthe source of p channel transistor 71 and the drain of n channeltransistor 72 are connected to a word line WL. The n channel transistor72 has its source connected to switch circuit 12, and switch circuit 12switches the low level of the word lines Vss and the potential Vss′ ofdummy GND line 30. Switch circuit 12 includes n channel transistors Tr10and Tr11 as shown in FIG. 16. To the gate of n channel transistor Tr10,the internally generated signal {overscore (φ)} is applied, and to thegate of n channel transistor Tr11, the internally generated signal φ isapplied. The drains of n channel transistors Tr10 and Tr11 are connectedto the source of n channel transistor Tr72 shown in FIG. 15. The nchannel transistor Tr10 has its source connected to the low level of theword lines Vss, and n channel transistor Tr11 has its source connectedto the potential Vss′ of dummy GND line 30.

At a crossing between word line WL and bit line BL, a memory cell 14 isconnected, and the bit lines BL and {overscore (BL)} are connected to asense amplifier 13. The power supply voltage Vcc and the potential Vssof dummy GND line 30 are applied to sense amplifier 13.

FIG. 17 is a time chart showing the operation of the embodiment of FIG.15. Referring to FIG. 15, the operation of the embodiment of FIG. 15will be described. When a row address signal shown in (b) of FIG. 17 isapplied to row decoder 11, a word line drive signal WD is applied fromrow decoder 11 to word driver 10. At this time, the internally generatedsignal φ applied to switch 12 is at the “L” level as shown in (d) ofFIG. 17, and {overscore (φ)} is at the “H” level. Therefore, switchcircuit 12 applies the low level of the word lines Vss to the source ofn channel transistor Tr72. After the row address strobe signal{overscore (RAS)} attains to the “L” level as shown in (a) of FIG. 17,after the lapse of time t4 but before the time t5 at which word liedrive signal falls to “L” level, the internally generated signal φattains to “H” level and {overscore (φ)} attains to “L” level.Consequently, switch circuit 12 applies the potential Vss′ of dummy GNDline 30 to the source of n channel transistor Tr72. As a result, asshown in (e) of FIG. 17, word line WL rises at first from the low levelof the word lines Vss to the potential Vss′ of the dummy GND line 30,and then boosted to the boosted voltage Vpp.

Meanwhile, since the low level of the word lines of a non-selected worddriver, not shown, is still at Vss, the non-selected word line ismaintained at the Vss level.

Since word line WL is first raised from the level of the low level ofthe word lines Vss to the potential Vss′ of dummy GND line 30 and thenraised to the boosted voltage Vpp as shown in (e) of FIG. 17, the loadof driving with the boosted power supply Vpp can be reduced and the wordline WL can be raised at high speed.

FIG. 18 is a block diagram showing a second embodiment in accordancewith the fourth aspect of the present invention. Different from theembodiment shown in FIG. 15 in which the low level of the word lines ofthe selected word driver only is switched to the potential Vss′ of thedummy GND line 30, in this embodiment shown in FIG. 18, the low level ofthe word lines of a word driver group including the selected word driveris switched to the potential Vss′ of the dummy GND line 30, so as toreduce the number of switch circuits. More specifically, there are aplurality of memory cell array blocks MC1, MC2 and MC3 respectivelyarranged between sense amplifiers SA1, SA2, SA3 and SA4 and word driversWD1 to WD3 are provided corresponding to memory cell array blocks MC1 toMC3, respectively. A word line drive signal is applied from row decoder11 to these word drivers WD1 to WD3. Word drivers WD1 to WD3 drive wordlines of the corresponding memory cell array blocks MC1 to MC3. Further,in order to switch the low level of the word lines of each word driverWD1 to WD3 between Vss and the potential Vss′ of the dummy GND line 30,switch circuits SW1 to SW3 are provided, each of which switch circuitsSW1 to SW3 is independently selected by block selecting-signals VS1 toVS3.

FIG. 19 is a schematic diagram showing an example of the switch circuitshown in FIG. 18. The switch circuits SW1 to SW3 shown in FIG. 18 eachinclude n channel transistors Tr12 and Tr13 and an inverter 14 as shownin FIG. 19. A block selecting signal BSi is applied to the gates of nchannel transistors Tr12 and Tr13, and the drains of these transistorsare commonly connected to the ground line VSi of word driver WDi. The nchannel transistor Tr12 has its source connected to the potential Vss′of dummy GND line 30, and n channel transistor Tr13 has its sourceconnected to the low level of the word lines Vss.

FIG. 20 is a time chart showing the operation of FIG. 18. Referring to(a) of FIG. 20, when a word driver WD2 is selected by block selectingsignal BS2, the ground line of word driver WD2 is raised from the lowlevel of the word lines Vss to the potential Vss′ of dummy GND line 30as shown in (b) of FIG. 20. Before the word line driving signal WD21from row decoder 11 rises to the “H” level at the timing shown in (c) ofFIG. 20, the word line WL21 and non-selected word line WL22 connected toword driver WD2 are raised to the potential Vss′ of dummy GND line 30from the low level of the word lines Vss, as shown in (d) of FIG. 20.When the word line driving signal WD21 is raised to “H”, only the wordline WL21 is raised to the boosted voltage Vpp. At this time, referringto (e) of FIG. 20, the non-selected word line WL22 is maintained at thepotential Vss′ of the dummy GND line, and therefore disturb refreshcharacteristic is not degraded.

Since block selection signals BS1 and BS3 are at the “L” level as shownin FIG. 20(f), word drivers WD1 and WD3 are not selected, the groundlines thereof are maintained at the low level of the word lines Vss asshown in (g) of FIG. 20, and word lines WL1 and WL3 are also maintainedat the low level of the word lines Vss as shown in (h) of FIG. 20.

In the embodiment shown in FIG. 18, memory cell array blocks MC1 to MC3divided by sense amplifiers SA1 to SA4 have been described as anexample. However, the low level of the word lines may be switched in anyother arbitrary blocks.

FIG. 21 is a time chart showing the operation of a third embodiment inaccordance with the fourth aspect of the present invention. In theembodiment shown in FIG. 18, when the “L” level of all the word lines inthe selected block attains to the potential Vss′ of dummy GND line 30,the disturb refresh characteristic of the memory cell connected to thenon-selected word line in the selected block is degraded. The embodimentshown in FIG. 21 addresses this problem. More specifically, referring to(a) of FIG. 21, in response to the rise of block selection signal BS2,an activating signal φ2 of the switch circuit attains to and kept at “H”only in the period t7 as shown in (b) of FIG. 21. As the activatingsignal φ2 rises, the “L” level of the word line in the selected blockattains to the potential Vss′ of the dummy GND line 30. After the lapseof time t1 from the rise of the block selecting signal BS2, a specificword driver WD2 is activated, and the selected word line WL21 is raisedto the boosted voltage Vpp level. When the activating signal φ2 fallsafter the lapse of time t2, the level of the non-selected word line WL22returns from the potential Vss′ of the dummy GND line 30 to the lowlevel of the word lines Vss. Accordingly, degradation of the disturbrefresh characteristic of the memory cell connected to the non-selectedword line in the selected block can be prevented.

FIG. 22 is a schematic diagram showing a first embodiment in accordancewith a fifth aspect of the present invention. This embodiment is acombination of embodiments of FIGS. 4 and 5, additionally including asustain circuit 15 for compensating level lowering of sub thresholdcurrent or the like cause by leakage, for example. To dummy GND line 30,n channel transistors Tr1 and Tr2 are connected similar to theembodiment of FIG. 4, and n channel transistor Tr3 and differentialamplifying circuit 8 shown in FIG. 5 are connected. A sustain circuit 15is further connected to the dummy GND line 30. Sustain circuit 15includes an oscillator 16 and a pumping circuit 17, and in response toan oscillating signal generated by oscillator 16, pumping circuit 17intermittently supplies the power supply voltage Vcc to the dummy GNDline 30.

The operation of the embodiment shown in FIG. 22 will be described. Atthe standby state, from a diode connected n channel transistor Tr1, thelevel of the threshold voltage Vthn of the n channel transistor Tr1 isprovided to the dummy GND line 30. In response to an oscillating outputfrom oscillator 16, the sustain circuit connected to the dummy GND line30 supplies, by means of the pumping circuit 17, the power supplyVoltage Vcc intermittently to the dummy GND line 30, so as to compensatefor the lowering of the level of the sub threshold current or the likecaused by the leakage.

In the active state, only during the sense amplifier operation in whichlarge current is consumed, n channel transistor Tr2 is renderedconductive, resistance of the sensing power supply line is lowered, andfloating of the dummy GND line 30 is controlled so as to increase thespeed of operation of the sensing operation. In the active period, otherthan the sense amplifier operating period, other than the senseamplifier operating period, differential amplifying circuit 8 isactivated so as to compensate for the floating of the dummy GND line 30caused by charges flowing in from the I/O line resulting from operationof the column circuitry and to maintain stable potential Vss′. By thiscombination, it becomes possible to realize lower power consumption andgeneration of stable potential Vss′ higher than the low level of theword lines at the dummy GND line 30.

FIG. 23 is a schematic diagram showing a first embodiment in accordancewith a sixth aspect of the present invention. In this embodiment shownin FIG. 23, when the sense drive line SN for activating a senseamplifier in the array circuit shown in FIG. 101 is at the “L” level, itis set to the potential Vss′ of the dummy GND potential which is higherthan the low level of the word lines. For this purpose, a comparingcircuit 8 and a dummy GND level generating circuit 19 is provided and,from the output of comparator circuit 8, the dummy GND potential Vss′ isgenerated from dummy GND level generating circuit 19 and applied tosense drive line SN. Comparing circuit 8 compares the level of sensedrive line SN with a reference voltage. A discharge transistor which isrendered conductive in response to an output from comparator circuit 8,for example, may be used as the dummy GND level generating circuit 19.

An activating signal is applied to comparing circuit 8 such that it isactivated only when the sense amplifier 2 is active. The reason why isthat the level of sense drive line SN is set to a desired prechargelevel at the time of precharging.

FIG. 24 is a time charge showing the operation of FIG. 23. As alreadydescribed with reference to FIG. 102 of the prior art, when the wordline WL rises to the boosted voltage Vpp as shown in (a) of FIG. 24, aninitial signal is read to the bit line pair BL, BL as shown in (e) ofFIG. 24. By sense amplifier activating signals S0 and S0 shown in (b)and (c) of FIG. 24, sense amplifiers 2 and 4 start sensing operation. Atthis time, comparing circuit 8 is activated by activating signal S0,compares the level of sense drive line SN with the reference voltage,and cause the dummy GND level generating circuit 19 to discharge untilthe level of sense drive line SN attains equal to the reference voltage.When the level of the sense drive line SN becomes equal to the referencevoltage, discharging operation of dummy GND level generating circuit 19stops, and potential drop of sense drive line SN stops. Consequently,the potential of sense drive line SN attains to the dummy GND potentialVss′ which is higher than the low level of the word lines, and thepotential of the “L” level of bit line BL attains to the dummy GNDpotential Vss′.

FIG. 25 shows a second embodiment in accordance with the sixth aspect ofthe present invention. In this embodiment, the dummy GND levelgenerating circuit 19 generating the dummy GND potential Vss′ isprovided separately, and when n channel transistor Tr14 is renderedconductive by sense amplifier activating signal S0, the potential ofsense drive line SN is forced to be discharged to the dummy GNDpotential Vss′. In the embodiment shown in FIG. 23, it is necessary toprovide a comparing circuit 8 and a dummy GND level generating circuit19 for each sense amplifier. However, in this embodiment shown in FIG.25, only one dummy GND level generating circuit 19 is necessary for anumber of sense amplifiers and what is necessary is to provide an nchannel transistor Tr14 for each sense amplifier. Therefore, thearrangement space can be reduced.

FIG. 26 shows a third embodiment in accordance with the sixth aspect ofthe present invention. In this embodiment, a path for discharging thesense drive line SN to the low level of the word lines and a path fordischarging the sense drive line to the dummy GND potential Vss′ areprovided. More specifically, to sense drive line SN, drains of n channeltransistors Tr15 and Tr16 are connected. The n channel transistor Tr15has its source connected to the dummy GND level generating circuit 19show in FIG. 25, and n channel transistor Tr16 has its source grounded.The n channel transistor Tr15 receives at its gate a second activatingsignal S02, and n channel transistor Tr16 receives at its gate a firstactivating signal S01.

FIG. 27 is a time chart showing the operation of the embodiment shown inFIG. 26. The operation of the embodiment of FIG. 26 will be describedwith reference to FIG. 27. At the time t8 when the sensing operationstarts, the first activating signal S01 attains to the “H” level asshown in (d) of FIG. 27, and n channel transistor Tr16 is renderedconductive. As a result, sense drive line SN is discharged to the lowlevel of the word lines and after the time t8, the first activatingsignal S01 attains to the “L” level, stopping discharging of n channeltransistor Tr16. At time t9, the second activating signal S02 attains tothe “H” level as shown in (e) of FIG. 27, n channel transistor Tr15 isrendered conductive, and sense drive line SN is discharged to the dummyGND potential Vss′. As a result, the sloop of the fall of the sensedrive line SN to the “L” level can be made steep, improving response.

More specifically, if n channel transistor Tr16 of FIG. 26 is notprovided and sense drive line SN is discharged to the dummy GNDpotential Vss′ immediately after the start of sensing operation only byn channel transistor Tr15, then the source-drain voltage of n channeltransistor Tr15 becomes lower, and discharging power of n channeltransistor Tr15 decreases, so that the fall becomes less steep. Bycontrast, when n channel transistor Tr16 is provided parallel to nchannel transistor Tr15 and the level of the sense drive line SN isdischarged to the low level of the word lines at the initial stage, alarge discharging power can be obtained because of the large potentialdifference, and therefore the sloop of fall can be made steep.

Referring to FIG. 26, the first and the second activating signals S01and S02 may be switched by simply using a delay circuit, or the signalsmay be switched by monitoring the change in the level of the bit line orthe level of the sense drive line SN.

FIG. 28 shows a fourth embodiment in accordance with the sixth aspect ofthe present invention. In this embodiment, an n channel transistor Tr17is connected in series between the ground and n channel transistor Tr 15shown in FIG. 26, so as to make higher the level of sense drive line SNby the threshold voltage of n channel transistor Tr17. Morespecifically, n channel transistor Tr17 has its gate connected to sensedrive line SN, its drain connected to the source of transistor Tr15, andits source grounded. In this embodiment also, at first transistor Tr16is rendered conductive by the first activating signal S0 to dischargesense drive line SN, and then transistor Tr15 is rendered conductive bythe second activating signal S02, and the potential of sense drive lineSN is discharged to reach the threshold voltage of transistor Tr17.Generally, the level of the sense drive line after the sensing operationis stabilized in the sense amplifier is neither discharged nor charged.Therefore, in order to stabilize the level thereof, what is necessary isonly to compensate for the leakage current in the sense amplifier. Byusing the diode connected transistor Tr17, compensation is realized.

In the embodiment shown in FIG. 28, the potential Vss′ can be obtainedby the threshold voltage of the diode connected transistor Tr17.Therefore, it is not necessary to separately provide the dummy GND levelgenerating circuit 19 such as shown in FIG. 26.

FIG. 29 is a schematic diagram showing a fifth embodiment in accordancewith the sixth aspect of the present invention. In this embodiment shownin FIG. 29, the level of the sense drive line SN and the referencevoltage are compared by a reference circuit 20, a discharge transistorTr16 is controlled by the comparison output such that the sense driveline SN is discharged, and discharging of discharge transistor Tr isstopped by transistor Tr26 when sensing operation is inactive.

Comparing circuit 20 is formed by a simple current mirror circuitincluding p channel transistors Tr21 to Tr23 and n channel transistorsTr24 and Tr25. More specifically, transistor Tr21 receives at its gatean activating signal {overscore (S0)}, and has its source connected tothe power supply Vcc and its drain connected to the sources oftransistors Tr22 and Tr23. Transistor Tr22 receives at its gate thereference voltage, transistor Tr23 has its gate connected to sense driveline Sn, and transistor Tr22 has its drain connected to the drain oftransistor Tr24, the drain of transistor Tr26 and the gate of transistorTr16 through a node B. Transistor Tr23 has its drain connected to thedrain and the gate of transistor Tr25 through a node A, transistor Tr24has its gate connected to the gate of transistor Tr25, and transistorsTr4 and Tr25 have their sources grounded. Transistor Tr16 has its drainconnected to sense drive line SN and its source grounded. TransistorTr26 receives at its gate the activating signal {overscore (S0)}, andhas its source grounded.

The operation will be described. Assume that the sense drive line SN ischarged to 1.25V and a reference voltage of, for example, 0.5V isapplied to the gate of transistor Tr22. Transistor Tr21 is renderedconductive when the “L” level activating signal {overscore (S0)} isapplied, and apply the power supply voltage Vcc to the sources oftransistors Tr22 and Tr23. At this time, since a potential of 1.25Vwhich is higher than the reference voltage has been applied to the gateof transistor Tr23, only a small current flows through transistor Tr23,and therefore node A cannot be charged. Accordingly, the potential atnode A lowers, reducing the current flowing into transistor Tr25.

Meanwhile, transistor Tr22 is rendered conductive as the referencevoltage is 0.5V, pulling the node B to the “H” level. At this time,since the activating signal S0 at the “L” level has been applied to thegate of transistor Tr26, transistor Tr26 is non-conductive. Therefore,the gate of discharging transistor Tr16 attains to the “H” level andrendered conductive to discharge the potential of sense drive line. Whenthe potential of node A lowers, transistor Tr25 becomes less conductive,causing the potential of node A to reach “H” level, the potential atnode B to “L”, and discharging transistor Tr16 stops discharging of thepotential of sense drive line SN.

FIG. 30 is a schematic diagram showing a seventh embodiment inaccordance with the sixth aspect of the present invention. Thisembodiment is a modification of the embodiment shown in FIG. 29, inwhich a negative potential is applied from negative potential generatingcircuit 9 to sources of transistors Tr24 and Tr25. The reason for thisis that the range of operation of the current mirror circuit 20 maypossibly be exceeded when the reference voltage is low, and that thelevel for comparison is set in the operational range of current mirrorcircuit 20 by setting the low level of the word lines of the currentmirror circuit 20 to a lower negative potential.

FIG. 31 is a schematic diagram showing an eighth embodiment inaccordance with the sixth aspect of the present invention. As comparedwith the embodiment shown in FIG. 29, in the embodiment shown in FIG.31, a level conversion circuit is provided between the gate oftransistor Tr23 and the sense drive line SN, so that even when thecomparison level is low, it can operate within the operational range ofcurrent mirror circuit 20. More specifically, transistor Tr23 has itsgate connected to the drain and the gate of transistor Tr27, transistorTr27 receiving at its source the power supply voltage Vcc through aresistor 21, and its source connected to the sense drive line SN. Sincetransistor Tr27 is diode connected, the potential of sense drive line SNhas its level shifted by the threshold voltage Vth of transistor Tr27and applied to the gate of transistor Tr23.

The reference voltage serving as the comparison level is set to avoltage higher than in the embodiment shown in FIG. 29. For example, itis set to 1.0V. The potential of the sense drive line SN of which levelhas been shifted is compared with the reference voltage by currentmirror circuit 20, and discharging transistor Tr16 is controlled by thecomparison output therefrom.

FIG. 32 is a schematic diagram showing a ninth embodiment in accordancewith the sixth aspect of the present invention, and FIG. 33 is aschematic diagram showing an embodiment in which the current sensor ofFIG. 32 is used as a comparing circuit.

Current sensor 31 includes, between the power supply voltage Vcc and theground, a path including a series connection of p channel transistorTr31 and n channel transistors Tr33, Tr35 and Tr37, and a path includinga series connection of a p channel transistor Tr32 and n channeltransistors Tr34, Tr36 and Tr38, and the sensor compares the currentsflowing through these two paths and provides a comparison output.

More specifically, as shown in FIG. 33, a reference voltage is input tothe gates of transistors Tr35 and Tr36, the drain and the gate oftransistors Tr37 and Tr38 are diode-connected respectively, andconnected to sense drive line SN. If the gate of transistor Tr37 is notconnected to the sense drive line SN, nodes A and B are both at thethreshold voltage Vth of respective transistors Tr37 and Tr38, as thetransistors Tr37 and Tr38 are both diode-connected. However, when a highpotential is applied from the sense drive line SN to the gate oftransistor Tr37, the potential at node A rises, current Ir1 flowingthrough node A decreases while current Ir2 flowing to node B increases.If the current Ir1 decreases, the potential at node C rises. To the nodeC, the gate of discharging transistor Tr16 is connected, and thereforedischarging transistor Tr16 is rendered conductive as the potential atnode C rises, discharging the potential of sense drive line SN. As thepotential of sense drive line SN is discharged and the potentialdecreases, the potential at node A decreases correspondingly, thecurrent Ir1 increases, while the current Ir2 decreases. When thepotential of sense drive line SN comes to be approximately the same asthe reference voltage, the node C attains to the “L” level, dischargingtransistor Tr16 is rendered non-conductive, and discharge stops.

FIG. 34 is a schematic diagram showing another example of the currentsensor. Current sensor 31 shown in FIG. 34 is a simplified version ofcurrent sensor 31 shown in FIG. 32, in which transistor Tr35 and Tr36are eliminated. The operation is approximately the same as the currentsensor 31 of FIG. 32.

FIG. 35 is a schematic diagram showing still another example of thecurrent sensor. Compared with the current sensor 32 shown in FIG. 34,the current sensor shown in FIG. 35 is connected to a voltage convertingcircuit including transistors Tr39, Tr41 as well as Tr40 and Tr42, sothat the current comparison in current sensor 31 is converted to voltagecomparison.

FIG. 36 is a schematic diagram of a current sensor adapted to havehysteresis characteristic. When the potential of sense drive line SN isdischarged and attains lower than the comparison voltage, the potentialof sense drive line SN may oscillate because of operations of othercircuits in which the potential of sense drive line SN once rises to behigher than the reference voltage and lowers to be lower than thereference voltage. In order to absorb such oscillation, in thisembodiment shown in FIG. 36, current sensor 32 is adapted to havehysteresis characteristic. More specifically, the comparison output ofcurrent sensor 32 is applied to the gate of discharging transistor Tr16as well as to a control circuit 40. When the comparison output fromcurrent sensor 32 is at the “H” level, control circuit 40 applies asignal of the “H” level to the gate of transistor Tr39. Transistor Tr39has its drain connected to the gate and the drain of transistor Tr38,and its source grounded. When the potential of sense drive line SN ishigher than the reference voltage, discharging transistor Tr16discharges the potential of sense drive line SN in accordance with thecomparison output of current sensor 32.

At this time, since control circuit 40 renders conductive transistorTr39, the current flowing at node B is the sum of currents flowingthrough transistors Tr38 and Tr39, and hence it is larger than thecurrent flowing through node A. When the potential of sense drive lineSN becomes lower than the reference voltage, the comparison output fromcurrent sensor 32 attains to the “L” level, and discharging transistorTr16 stops discharging. In response to the comparison output attaining“L”, control circuit40 renders non-conductive the transistor Tr39.Accordingly, the current Ir1 flowing through node B becomes smaller thanthe current Ir2 flowing through node A, the potential at node B rises,and comparison level rises substantially, generating hysteresis in thecomparison level. When the potential at sense drive line SN becomeslower than the reference voltage, the comparison level becomes higherbecause of the hysteresis characteristic, and hence oscillation of thelevel of sense drive line SN can be prevented.

FIG. 37 is a block diagram showing an example in which the invention inaccordance with a sixth aspect is applied to a test circuit for testingthe data retention time of the memory cell.

The above described embodiments are directed to prevent sub thresholdleak of the word line transistor by setting higher the low levelpotential of the bit line than the low level of the word lines, bysetting the potential of the sense drive line SN higher than the lowlevel of the word lines by the potential Vss′.

However, when data retention time of the memory cell is to be tested,sometimes it is desirable to test under the condition in which subthreshold leak is likely. Therefore, in the embodiment shown in FIG. 37,in the test mode, sense drive line SN is set to the low level of theword lines and in the normal mode, the potential of the sense drive lineis set to the potential Vss′ of the dummy GND line.

More specifically, current sensor 31 compares the reference voltage withthe potential of sense drive line SN, and provides the comparison outputto control circuit 41. An activating signal S0 and a test signal areapplied to control circuit 41. Drains of transistors Tr41 and Tr42 areconnected to the sense drive line SN. To the source of transistor Tr41,the dummy GND potential Vss′ is applied from the dummy GND levelgenerating circuit 19 shown in FIG. 25. Transistor Tr42 has its sourcegrounded. When activating signal S0 is applied, control circuit 41applies a signal at “H” level to the gate of transistor Tr41, and at thetime of testing, applies a signal at the “H” level to the gate oftransistor Tr42.

Therefore, in normal use, when it is determined by the current sensor 31that the potential of sense drive line SN is higher than the referencevoltage and the activating signal S0 is applied, control circuit 41renders conductive the transistor Tr41 and discharges the sense driveline SN to the dummy GND potential Vss′. At the time of testing, whenthe test signal is applied to control circuit 41, control circuit 41renders conductive the transistor Tr42 and discharges the sense driveline SN to the ground level. Therefore, the sub threshold leak currentof word line transistor is increased and floating of the level of thenon-selected word line caused by the array noise is increased.Therefore, sub threshold leak current of the word line transistor isincreased. Accordingly, the retention time characteristic of the memorycell element represented by the disturb refresh characteristic isdegraded, and hence time necessary for evaluating the characteristic canbe reduced.

According to this embodiment, as compared with the prior art, retentioncharacteristic of the memory element can be surely improved by thedisturb test. Therefore, if the memory cell satisfies the targetretention time with this embodiment applied, it means that the retentiontime characteristic by the disturb test is surely improved. Therefore,the time necessary for disturb refresh or the like can be reduced. Sinceincrease of the time for testing with respect to refreshing, especiallyincrease of time for testing with respect to disturb refresh has come tobe a serious problem. Therefore, reduction in test time attained by thepresent embodiment is of significant importance.

FIG. 38 shows an example in which the embodiment in accordance with asixth aspect is applied to each memory block. In the example shown inFIG. 38, a plurality of dummy GND lines 51 are provided on a memoryblock including a number of memory cells (not shown), and a dummy GNDlevel generating circuit 19 is provided oputside the memory block 50.The dummy GND potential Vss′ is supplied from dummy GND generatingcircuit 19 to each dummy GND line 51. Discharging transistors Tr16 arearranged corresponding to dummy GND lines 51, respectively, and theactivating signal S0 is applied to the gate of each of the transistors.When the activating signal S0 attains to the “H” level, the dummy GNDpotential Vss′ generated from the dummy GND generating circuit 19 issupplied to sense drive line SN.

FIG. 39 shows another example of the invention in accordance with thesixth aspect applied to each memory block. In this example shown in FIG.39, the embodiment of FIG. 26 is applied. More specifically, thecomparing circuit 8, the dummy GND level generating circuit 19 andtransistor Tr15 are provided outside the memory block 50. When anactivating signal S0 is applied, comparing circuit 8 compares thepotential of sense drive line SN with the reference voltage, rendersconductive the transistor Tr16 in accordance with the comparison output,and discharges the sense drive line SN to the ground level. The outputfrom comparing circuit 8 is delayed by a delay circuit 81, and after thelapse of a prescribed time period, transistor Tr15 is renderedconductive, discharging the potential of sense drive line SN to thedummy GND potential Vss′.

FIG. 40 shows a still another example of the present invention inaccordance with the sixth aspect applied to each memory block. On memoryblock 50, the dummy GND line 31 and the ground line 52 are arrangedalternately, and between the lines 51 and 52, discharging transistorsTr14 and Tr16 are arranged. Outside the memory block 50, comparingcircuit 8, delay circuit 81 and dummy GND level generating circuit 19are arranged. Comparing circuit 8 compares the reference voltage withthe potential of the sense drive line SN in response to the activatingsignal S0, and in accordance with the comparison output, dischargingtransistor Tr16 is rendered conductive and sense drive line SN isdischarged to the low level of the word lines. The comparison output isdelayed by the delay circuit 81, and after the lapse of a prescribedtime period, transistor Tr14 is rendered conductive and the sense driveline SN is discharged to the potential Vss′ of the dummy GND line 51.

FIG. 41 shows a still another example of the present invention inaccordance with the sixth aspect applied to each memory block. Theexample shown in FIG. 41 is a modification of the embodiment shown inFIG. 28. As in the example of FIG. 40, ground lines 52 and 53 arearranged alternately on memory block 50, and between these lines,discharging transistors Tr16 and Tr15 and a diode-connected transistorTr17 are arranged. In accordance with the comparison output fromcomparing circuit 8, discharging transistor Tr16 is discharged to thelow level of the word lines, after the lapse of a prescribed time perioddetermined by the delay circuit 81, discharging transistor Tr15 isrendered conductive, and the potential of the sense drive line SN isdischarged by the threshold voltage of transistor Tr17.

FIG. 42 is a schematic diagram showing a first embodiment in accordancewith a seventh aspect of the present invention. FIG. 42 shows memorycell blocks, sense amplifier portion and sense amplifier drive circuitof a dynamic RAM, which includes word lines WL, bit line pairs BL,{overscore (BL)}, memory cells MC, transfer gates BSA and BSB of sharedsense amplifier, control signal BLI thereof, a p channel sense amplifierPSA, a p channel sense amplifier drive line SP, an n channel senseamplifier NSA, an n channel sense amplifier drive line SN, a bit lineequalizing circuit EQ, an equalizing control circuit PLEQ, and aswitching circuit IOSW for the I/O bus. Further, it includes the dummyGND level generating circuit 19 and a power supply voltage loweringcircuit 60 which are features of the present embodiment. The powersupply voltage lowering circuit 60 provides a lowered voltage Vcc′ whichis provided by lowering the power supply voltage Vcc, and which loweredvoltage Vcc′ is supplied to the p channel sense amplifier drive line{overscore (SP)} through switch circuit SW10. The dummy GND levelgenerating circuit provides the dummy GND potential Vss′ which is higherthan the low level of the word lines, and the dummy GND potential Vss′is supplied to the drive line SN of n channel sense amplifier 2 throughswitch circuit SW11. Switch circuit SW10 is rendered conductive by asignal {overscore (SU/L)} for controlling the sense amplifier drive line{overscore (SP)}, and switch circuit SW11 is rendered conductive bysignals SU/L and SFU/L controlling the sense drive line SN of n channelsense amplifier 2.

FIG. 43 is a time chart showing the operation of the embodiment shown inFIG. 42. FIG. 43 shows main clocks representing internal operation ofthe dynamic RAM shown in FIG. 42. More specifically, the time chartshows external inputs {overscore (RAS)}, {overscore (CAS)} and An, aninternal low address signal RAn, internal column address signal CAn, rowpredecode signals Xi, j, k, column predecode signals Yi, j, k, a masterrow signal φx, a decode signal thereof, which is a trigger signal φx1 ofthe word line WL, a column enable signal {overscore (CE)} provided afterthe end of sensing operation, a signal CSL which is a column selectionsignal and used for connecting the I/O bus to the sense amplifier, and adata output Dout.

The operation of the embodiment shown in FIG. 42 will be described withreference to FIG. 43. At first, transfer gates BSA and BSB are bothconductive, memory blocks 1 a and 1 b are connected to a sense amplifierband 2 a, and bit line pair BL, {overscore (BL)} is precharged to VBL(=(Vcc′+Vss′)/2) as shown in FIG. 43(m) by the equalizing circuit 3.When memory cell block la is selected, memory cell block 1 b isdisconnected from sense amplifier band 2 a by transfer gate BSB.

Thereafter, when word line WL rises to “H” as shown in FIG. 43(l), datais read from memory cell MC to bit line BL and transferred to senseamplifiers 2 and 4. When n channel sense amplifier 2 is activated,switch circuit SW11 is rendered conductive by the signals SFU/L, SU/L asshown in FIG. 43(n), and the dummy GND potential Vss′ generated by dummyGND level generating circuit 19 is supplied through switch circuit SW11to sense drive line SN. When p channel sense amplifier 4 is activated,switch circuit SW10 is rendered conductive by the signal {overscore(SU/L)} shown in FIG. 43(p), and the voltage Vcc′ lowered by the powersupply lowering circuit 60 is supplied through switch circuit SW10 to pchannel sense drive line SP. By the sense amplifiers 2 and 4, thepotentials of the bit line pair BL and {overscore (BL)} are amplified toVcc′ and Vss′, respectively.

As already described with reference to the embodiments above, the effectobtained by setting the low level bit line to the dummy GND potentialVss′ will be described in detail in the following.

FIGS. 44 and 45 are illustrations showing the effects of the presentinvention.

(1) It Becomes Stable Against Disturb Refresh

The bit lines of the selected block are amplified to the potentials Vcc′and Vss′, and in the non-selected memory cells connected thereto, thegate potential of memory cell transistor MT is at Vss=0V, the bit lineBL is Vss′>0V, and the potential of the storage node is at the “H” levelpotential of Vcc′, as shown in FIG. 44. In this case, the sub thresholdcurrent (denoted by the arrow in FIG. 44) across the memory celltransistor MT can be significantly reduced as compared with theconventional example in which the potentials of the bit line BL and ofthe word line WL are both at 0V.

(2) The Threshold Voltage VTM of Memory Cell Transistor MT can be SetLow, Improving Reliability

As described in (1) above, since it can be made strong against disturbrefresh, the threshold voltage VTM of memory cell transistor MT can beset lower than in the prior art. Therefore, it becomes possible to lowerthe boosted voltage Vpp of the word line (the value Vpp must satisfyVpp>Vcc′+VTM in order to write data of the “H” level sufficiently highto the memory cell), improving reliability of the transistor.

(3) The Boosted Voltage Generating Circuit Becomes Unnecessary

Conventionally, memory cell substrate or well potential must be set to anegative potential in view of minority carrier injection. However, inthe present invention, the low level of the memory cell is the potentialVss′, and the potential of the substrate (well) is the low level of theword lines Vss. Therefore, viewed from the memory cell, the substrate issubstantially set to a bias potential of a negative voltage. Since theboosted voltage generating circuit becomes unnecessary, powerconsumption can be reduced.

(4) The Triple Well Structure Becomes Unnecessary

As described in ISSCC 89 Digest of Technical Papers pp. 248-249, when aP substrate is used, a triple well must be employed in order to set thewell potential to a negative potential of Vbb for preventing injectionand to set the peripheral circuits to the low level of the word linesVss so as to improve performance of the transistor, and therefore thenumber of programming steps must be increased. However, in the presentinvention, since the “L” level of the bit lines and the memory cell inthe memory cell array portion are set to the dummy GND potential Vss′and the well potential is set to the low level of the word lines, the“L” level of the peripheral circuitry and the well potential can be bothset to the low level of the word lines Vss, whereby the function of theaforementioned triple well structure can be realized by a common twinwell structure such as shown in FIG. 45.

FIGS. 46 to 50 are schematic diagrams for generating various clocksignals shown in FIG. 42. More specifically, FIG. 46 shows a circuit forgenerating row predecoder outputs Xi, j, k, FIG. 47 shows a circuit forgenerating the master row decode signal φxi, FIG. 48 shows a circuit forgenerating the word line driving signal, FIG. 49 shows a circuit forgenerating the column SFU/L signal, and FIG. 50 shows a circuit forgenerating the column selection signal. As shown in FIG. 46, internaladdress signals A0 to A3 are applied to a row address buffer 61, rowaddress signals RA0, {overscore (RA0)} to RA3, {overscore (RA3)} areprovided, of which row address signals RA2, {overscore (RA2)}, RA3,{overscore (RA3)} are applied to a row predecoder 62 and decoded intorow predecode signals X1, X2, X3 and X4. Row address signals RA0,{overscore (RA0)}, RA1 and {overscore (RA1)} are applied to a subdecoder 63 shown in FIG. 47. To sub decoder 63, the master row signal φxhas been applied, and in response to the row address signals RA0,{overscore (RA0)}, RA1 and {overscore (RA1)}, sub decoder 63 selects themaster row signal φx and provides φx1 to φx4.

The row predecode signals X1 to X4 shown in FIG. 46 are applied to a rowdecoder 64 shown in FIG. 48, and any of the signals φx1 to φx4 of FIG.47 is applied to the gate of the selection gate Tr51 of FIG. 48. Whenthe selection gate Tr51 is rendered conductive by the signal φx1, thedecoded output of row decoder 64 is output as the word line drivingsignal through a word line driver consisting of n channel transistorTr54 and p channel transistor Tr55. The master row signal φx is delayedby a delay circuit 67 shown in FIG. 49 and applied to one input and ofan AND gate 65. The row predecode output X1 or X3 and X2 or X4 areapplied to the other input end of the AND gate 66 through an OR gate 65,and from the AND gate 66, the signal SFU/L is provided. As shown in FIG.50, column predecode signals Yi, j, k, l are provided from a columnpredecoder, not shown, applied to a column decoder 69, and the outputthereof is inverted by an inverter 68 and output as the CSL signal.

The operations of the circuits for generating the various clock signalsshown in FIG. 46 to 49 will be briefly described with reference to FIG.43.

As shown in (c) of FIG. 43, internal address signal An is applied toaddress buffer 61 and output from address buffer 61 as a row addresssignal RAn as shown in (d) of FIG. 43, and applied to row predecoder 62.From row predecoder 62, a predecode signal Xi is provided as shown in(f) of FIG. 43. As shown in (j) of FIG. 43, from master row signal φxand row address signal RAn, sub decoder 63 provides signals φx1 to φx4as shown in (k) of FIG. 43. Row decoder 64 shown in FIG. 48 performsdecoding operation in accordance with the row predecoder outputs Xi, j,k, and selection gate Tr51 is rendered conductive in response to thesignal φxi and provides the word line driving signal shown in (l) ofFIG. 43. The master row signal φx is delayed by delay circuit 67, an ORof the row predecode output X1 or X3 and X2 or X4 is obtained by OR gate65, an AND of the output from OR gate 65 and the output from delaycircuit 67 is provided by AND gate 66, and the signal SFU/L is output asshown in (n) of FIG. 43. Referring to FIG. 50, column decoder 69provides the AND of column address signals Yi to Yl, the output thereofis inverted by inverter 68 and output as the CSL signal.

FIG. 51 is a schematic diagram showing a second embodiment in accordancewith the seventh aspect of the present invention. In the embodiment ofFIG. 51, the transfer gates BSA and BSB shown in FIG. 42 are replaced byp channel transistors Tr61 to 64, the voltage lowering circuit 60 shownin FIG. 42 is eliminated, and to the drive line {overscore (SP)} of pchannel sense amplifier 4, the power supply voltage Vcc′ is appliedthrough transistor Tr65. Transistors Tr61 and Tr62 are controlled bycontrol signal BLI′_(L), while transistor Tr63 and Tr64 are controlledby control signal BLI′_(R). The equalizing circuit shown in FIG. 42 isnot shown in FIG. 51.

FIG. 52 is a time chart showing the operation of the embodiment show inFIG. 51. The operation of the embodiment shown in FIG. 51 will bedescribed with reference to FIG. 52. At the time of precharging, controlsignals VLI′_(L) and VLI′_(R) are at the “L” level, transistors Tr61 toTr64 are rendered conductive, and the bit lines BL and {overscore (BL)}are precharged to the potential of VBL. When the left side block isselected, the control signal BLI′_(R) attains to “H” level, andtransistors Tr63 and Tr64 are rendered non-conductive. Then, referringto (a) of FIG. 52, the word line WL rises to the potential of Vpp, anddata is read from memory cell 1 to the bit line pair BL, {overscore(BL)} shown in (b) of FIG. 52. The read data is amplified by n channelsense amplifier 2 and then by p channel sense amplifier 4. At this time,the drive line SN of n channel sense amplifier 2 is connected to theground level Vss through switch 71, while drive line {overscore (SP)} ofp channel sense amplifier 4 is connected to the power supply voltageVcc′ through transistor Tr65. Since BLI′_(L) is at the low level of theword lines Vss, the “H” level of the bit line pair BL, {overscore (BL)}is at the level of the power supply voltage Vcc′, while “L” level isfloating higher than the low level of the word lines Vss by thethreshold voltage Vth of transistors Tr61 and Tr62. More specifically,it is at the level Vss′ higher than the low level of the word lines Vssby the threshold voltage Vth. In the embodiment shown in FIG. 51, thedummy GND level generating circuit 19 shown in FIG. 42 is not necessary.Since the sense amplifiers fully swing to the low level of the wordlines Vss, the sensitivity can be improved.

FIG. 53 is a schematic diagram showing a third embodiment in accordancewith the seventh aspect of the present invention. This embodiment is animprovement over that of FIG. 51, which addresses the problem of theembodiment of FIG. 51 that when the sense drive lines SN and {overscore(SP)} are equalized, the potential of bit line pair BL, {overscore (BL)}attains not to VBL=(Vcc′+Vss′)/2 but to Vcc′/2. More specifically, inthis embodiment, sense drive line SN is connected to the drains oftransistors Tr65 and Tr66 through switch circuit 71, transistor Tr65receives at its gate the control signal DC and has its source grounded.The transistor Tr66 receives at its gate the control signal DC and atits source, the potential Vss′ from the dummy GND level generatingcircuit 19.

FIG. 54 is a time chart showing the operation of the embodiment of FIG.53. The operation of the embodiment shown in FIG. 53 will be describedwith reference to FIG. 54. At the start of sensing operation, when rowaddress strobe signal {overscore (RAS)} of (a) of FIG. 54 attains to the“L” level, this signal is delayed and provided as the control signal DC.More specifically, when control signal DC rises to Vcc′ as shown in FIG.54(d), transistor Tr65 is rendered conductive, and sense drive line SNattains to the low level of the word lines Vss level through switchcircuit 71. Namely, sense amplifier 2 operates with the potentialdifference between the low level of the word lines Vss and the powersupply potential Vcc′, resulting higher sensitivity. Then, after thelapse of a prescribe time period, control signal {overscore (DC)} risesto Vcc′, so that transistor Tr66 is rendered conductive, the potentialVss′ is applied from dummy GND level generating circuit 19 to sensedrive line SN, and the precharge potential of sense amplifier 2 becomesequal to the bit line potential VBL. More specifically, in theembodiment shown in FIG. 53, at the initial state of sensing operation,the sense drive line SN operates with the potential difference betweenthe low level of the word lines and Vcc′, and after the lapse of aprescribed time period, it operates between the potential Vss′ which ishigher than the low level of the word lines Vss and the power supplyvoltage Vcc′, as shown in (f) of FIG. 54.

FIG. 55 is a schematic diagram showing a fourth embodiment in accordancewith the seventh aspect of the present invention. In the embodimentshown in FIG. 55, the potential of sense drive line SN is switchedbetween the low level of the word lines Vss and the dummy GND potentialVss′ by means of transistors Tr67 and Tr68. Transistor Tr67 is renderedconductive by control signal SFU/L, while transistor Tr68 is controlledby control signal SU/L. Transistor Tr67 has its source connected to thelow level of the word lines Vss, and transistor Tr68 has its sourceconnected to the dummy GND level generating circuit 19.

FIG. 56 is a time chart showing the operation of the embodiment shown inFIG. 55. As shown in FIG. 56(c), at the start of sensing operation, thesignal SFU/L attains to the “H” level, transistor Tr67 is renderedconductive, and the sense drive line SN attains to the low level of theword lines Vss. Therefore, sense amplifier 2 operates between the lowlevel of the word lines Vss and the power supply voltage Vcc′, resultingin higher sensitivity at the initial stage of sensing and faster sensingoperation. Referring to (e) of FIG. 56, before the “L” level potentialof the bit line BL attains to the dummy GND level Vss′, the signal SFU/Lattains to the “L” level and the control signal SU/L attains to the “H”level as shown in FIG. 56, so that transistor Tr68 is renderedconductive, and the dummy GND level Vss′ is applied to sense drive lineSN, preventing overswing of bit lines BL and {overscore (BL)}.

FIG. 57 is an illustration showing the principle of an eighth aspect ofthe present invention. In the eighth aspect of the present invention,the dummy GND level higher than the low level of the word lines by apredetermined potential is set as in the first aspect of the presentinvention shown in FIG. 1(b), and in addition, an internal power supplyvoltage Int.Vcc which is lower than the external power supply voltageExt.Vcc by a predetermined potential is generated, so that the ICoperates between the internal power supply voltage Int.Vcc level and thedummy GND level. The IC operational voltage is selected to improve dataretention characteristic of the DRAM memory cells, and other potentialmay be set for other circuit portions.

FIG. 58 is an illustration showing sub threshold leak current of theword line with respect to the eighth aspect of the present invention.

In the state shown in FIG. 57, the operational voltage of the bit linesystem including the memory cells is between the internal power supplyvoltage Int.Vcc and the dummy GND. In other words, the amplitude levelof the “L” level side of the bit line is the dummy GND. The non-selectedlevel of the word line is the GND level. Consequently, the non-selectedlevel of the word line is made lower relative to the “L” level of thebit line, reducing the sub threshold leak current.

This will be described with reference to FIG. 58. FIG. 58 shows subthreshold leak of the word line, in which the abscissa represents thegate voltage and the ordinate represents the Log value of the leakcurrent. The sub threshold leak current characteristic of the word linewhen substrate potential is not applied is represented by a, and thelevel of the junction leak current is represented by d. If the gatevoltage is 0V, the sub threshold leak current is maintained smaller thanthe junction leak current. When a negative substrate potential isapplied, it changes as shown by b and further decreases. However,actually, the characteristic changes as shown by c dependent on thestate of application of voltage between the source-drain and increase intemperature, and therefore the sub threshold leak current when the gatevoltage is 0V is degraded to B. In this state, there is little marginwith respect to the junction leak current, and therefore it is highlypossible that it becomes larger than the junction leak current shown byC, responding to small gate potential noise with excessively highsensitivity. However, at this time, if the gate potential is setrelatively negative by applying the present invention, the sub thresholdleak current can be made sufficiently small as shown by D. Therefore,the substrate potential can also be set shallower. For example, if asubstrate potential of −80 mV is applied, the leak current can bereduced by one order of magnitude.

FIG. 59 is a schematic diagram showing a first embodiment in accordancewith the eighth aspect of the present invention. Referring to FIG. 59, ap channel transistor Tr71 has its source connected to the line of theexternal power supply voltage Ext.Vcc, its gate connected to an outputof a comparing circuit 82, its drain connected to a comparing input endof comparing circuit 82, and from the drain, the internal power supplyvoltage Int.Vcc is provided. Comparing circuit 82 compares the internalpower supply voltage Int.Vcc with a reference voltage A, and lowers theexternal power supply voltage Ext.Vcc to internal power supply voltageInt.Vcc. An n channel transistor Tr72 has its drain connected to theexternal GND, its gate connected to an output of a comparing circuit 82,and its source provides the dummy GND, and is connected to a comparinginput end of a comparing circuit 83. Comparing circuit 83 compares thedummy GND with the reference voltage, and generates the dummy GND levelwhich is slightly higher than the external GND, from n channeltransistor Tr72.

FIG. 60 shows IC operational voltage of the embodiment shown in FIG. 59,and FIG. 61 shows changes in the reference voltage of the embodimentshown in FIG. 59.

As in the embodiment shown in FIG. 59, newly setting the dummy GND whilemaintaining constant the internal power supply voltage Int.Vcc furtherreduces the operational voltage of the memory cell especially when theDRAM memory array circuit is operated. This leads to reduction of theamount of charges to be stored in the memory cells, causing undesirabledegradation of retention characteristic. Therefore, in that case, theinternal power supply voltage Int.Vcc may be shifted in accordance withthe amount of shift of the dummy GND from the operational range shown in(a) of FIG. 60 to (b) or (c) of FIG. 60 in order to ensure sufficientamount of charges to be stored.

By this structure, the amount of charges stored in the memory cell canbe maintained, and the sub threshold leak current can be suppressed bysuch a structure as shown in FIG. 59. In addition, the substrate biaspotential can be reduced and the junction leak current can also besuppressed. Therefore, retention characteristic of the memory cell canbe significantly improved. This can be implemented quite simply, only bycontrolling the reference voltages A and B applied to the internal powersupply voltage generating circuit and to the dummy GND generatingcircuit such that the difference therebetween is kept constant. Morespecifically, referring to (a) to (c) of FIG. 61, the reference voltagesA and B should be set such that the difference Vb provided bysubtracting the reference voltage A for generating the internal powersupply voltage Int.Vcc from the reference voltage B for generating thedummy GND is kept unchanged. However, as can be readily understood fromthe embodiment shown in FIG. 59, the potentials of the internal powersupply voltage Int.Vcc and of the dummy GND can be freely adjusted bychanging the amount of shifts of the reference voltages A and B.

FIG. 62 is a schematic diagram showing an example of the referencevoltage generating circuit for generating the reference voltage shown inFIG. 59. Referring to FIG. 62, a p channel transistor Tr81 has itssource connected to the external power supply voltage Ext.Vcc line, andits drain connected to the drain of an n channel transistor Tr82 and tothe gate of a p channel transistor Tr83. The n channel transistor Tr82has its source grounded, and the p channel transistor Tr83 has itssource connected to the gate of p channel transistor Tr81, the gate of pchannel transistor Tr85 and to one end of a resistor R1. The resistor R1has the other end connected to the external power supply voltage Ext.Vccline.

The p channel transistor Tr83 has its drain connected to the gate of nchannel transistor Tr82 and to the gate and the drain of n channeltransistor Tr84. The n channel transistor Tr84 has its source grounded,and p channel transistor Tr85 receives at its source the external powersupply voltage Ext.Vcc. The p channel transistor Tr85 has its draingrounded through resistors R2 and R3. From the node between the drain ofp channel transistor Tr85 and the resistor R2, the reference voltageVref1 is provided, and from the node between resistors R2 and R3, thereference voltage Vref2 is provided.

In the reference voltage generating circuit shown in FIG. 62, n channeltransistors Tr82 and Tr84 and p channel transistor Tr83 constitute acurrent mirror circuit, and therefore the current I flowing through pchannel transistor Tr81 comes to be equal to the current I flowingthrough resistor R1. At this time, the amount of current can berepresented as I=Vthp/R1, as the threshold value Vthp of p channeltransistor Tr81 and the potential drop across resistor R1 are the same.If p channel transistors Tr81 and Tr85 have the same size, e.g. as thesame channel width, the same current flows through respectivetransistors. However, when p channel transistor Tr85 is adapted to havethe current drivability n times that of p channel transistor Tr81 bychanging, for example, the channel width of p channel transistor Tr85,the amount of current flowing through p channel transistor Tr85 can berepresented as nI. Therefore, the currents flowing through resistors R2and R3 can be represented as nI, and reference voltages Vref1 and Vref2have the following values.Vref1={(n×Vthp)/R 1}×(R 2+R 3)Vref2={(n×Vthp)/R 1}×R 3

At this time, the difference between the reference voltages Vref1 andVref2 is represented byVref1−Vref2={(n×Vthp)/R 1}×R 2

Namely, the difference between the reference voltages Vref1 and Vref2 isrepresented as the potential drop of resistor R2 caused by the currentflowing through p channel transistor Tr85, and therefore it is possibleto shift while the difference between the reference voltages Vref1 andVref2 is maintained, only by changing the resistance of resistor R3.

FIG. 63 shows an improvement of the reference voltage generating circuitshown in FIG. 62. When the internal power supply voltage Int.Vcc and thedummy GND are generated based on the reference voltages generated fromthe reference voltage generating circuit shown in FIG. 62 and the memorycell is operated in accordance with the generated voltages, it becomesnecessary to shift the ½ Vcc level of the ½ Vcc bit line prechargemethod currently used in the DRAM as well. Therefore, in the exampleshown in FIG. 63, the resistor R2 is replaced by resistors R21 and R22by dividing the resistance to ½. From the node between the resistors R21and R22, a new reference voltage (Vref1÷Vref2)/2 is generated. When the½ Vcc generating circuit is configured based on the new referencevoltage, the ½ Vcc level potential can be readily generated, andtherefore even when the internal power supply voltage Int.Vcc and thedummy ground GND change, the change can be followed.

FIG. 64 is a schematic diagram showing a still another improvement ofthe reference voltage generating circuit shown in FIG. 62. Theembodiment of FIG. 64 is adapted such that the dummy GND and theinternal power supply voltage Int.Vcc generated based on the referencevoltages Vref1 and Vref2 set in the circuit configuration shown in FIG.62 can be shifted provisionally. More specifically, it is used when thedummy GND is set at approximately the same potential as the external GNDprovisionally, and in this example, an n channel transistor Tr86 isconnected parallel to the resistor R3, and to the gate of n channeltransistor Tr86, a test signal is applied.

FIG. 65 is a diagram of waveforms showing the operation of the circuitshown in FIG. 64. Generally, the test signal applied to the gate of nchannel transistor Tr86 is at the inactive state of “L” level, andreference voltages Vref1 and Vref2 are set at prescribed potentials asalready described with reference to FIG. 62. If desired, for examplewhen it is desired to evaluate the retention characteristic of thememory cell with the sub threshold leak current accelerated with thestorage capacity of the memory cell maintained, it is possible to lowerthe potential while the potential difference between the internal powersupply voltage Int.Vcc and the dummy GND is kept constant, byshort-circuiting the reference voltage Vref2 to the external GND, byactivating the test signal to “H” level. After the end of the test, thetest signal is again inactivated, and the potentials of referencevoltages Vref1 and Vref2 can be set to the previous potentials.

FIG. 66 is a schematic diagram showing a still further example of thereference voltage generating circuit shown in FIG. 66. In the referencevoltage generating circuit shown in FIG. 66, the resistor R2 of FIG. 62is replaced by n resistors R211, R212, . . . , R21 n connected inparallel, and resistor R3 is replaced by m resistors R311, R312, . . . ,R31 m connected in parallel. Corresponding to respective resistors R211,R212, . . . , R21 n, fuses 911, 912, . . . , 91 n are provided, andcorresponding to the resistors R311, R312, . . . , R31 m, fuses 921,922, . . . , 92 m are provided. By changing respective resistance valuesby blowing off the fuses 911, 912, . . . , 91 n and 921, 922, . . . , 92m by laser trimming or other means, the reference voltages Vref1 andVref2 can be adjusted. A voltage determined by the voltage drop derivedfrom the resistances of resistor R311, R312, . . . , R31 m is providedas reference voltage Vref2, and as a voltage between the referencevoltages Vref1 and Vref2, a voltage determined by the voltage dropacross resistors R211, R212, . . . , R21 n and R311, R312, . . . , R31 mis provided. The resistance value increases as larger number of fusesare blown off, and thus the voltage value can be freely adjusted.

FIG. 67 shows a still another example of the reference voltagegenerating circuit. In the reference voltage generating circuit shown inFIG. 67, n channel transistor Tr911, Tr912, . . . , Tr91 n, Tr921,Tr922, . . . , Tr92 m are connected in series to fuses 911, 912, . . . ,91 n, 921, 922, . . . , 92 m, respectively. Before blowing off the fuse,the corresponding transistor is rendered conductive so as to adjust thevalues of the reference voltages Vref1 and Vref2, test result isconfirmed, and then the fuse is blown off to provide the desiredvoltage. In this case, on resistance of each of n channel transistorsTr911, Tr912, . . . , Tr91 n, Tr921, Tr922, . . . , Tr92 m must also beconsidered.

The embodiment shown in FIG. 67 can be applied not only to testing butalso special applications. For example, recently a method has beenproposed in which refreshing of the DRAM memory cell is controlled sothat the interval between refreshing is made longer than specified inthe standard specification so as to reduce current consumption inrefreshing operation which is the bottleneck of the DRAM, specially whenthe DRAM is not used for a long period of time. At this time, if furtherreduction of current consumption is desired, the operational voltage maybe reduced in addition to widening of the interval between refreshingoperations. In such a circumstance, the circuit shown in FIG. 67 may beused to shift the operational voltage to the optimal state, so as toreduce current consumption.

FIG. 68 is a schematic diagram of an embodiment in which powerconsumption is reduced at the time of refreshing operation, when thechip is not used.

For example, in a DRAM, in normal operation, the subthreshold leakcurrent of the word line is increased by disturbing during normaloperation. Accordingly, it is assumed that the operational voltage ofthe memory cell is modified by the circuit of the present invention andthe power supply for the constant potential is set slightly higher thanthe power supply of the lower potential side. Thereafter, the chip iskept inoperative for a long period of time and only the refreshingoperation is carried out.

In that case, there is not much disturbing and therefore increase of theleak current because of the disturbing is not a serious problem. Then,by lowering the lower potential side of the operational voltage of thememory cell, the electric field across the junction of the memory cellis made smaller, whereby increase of the leak current because of thejunction leak can be reduced.

The embodiment shown in FIG. 68 is adapted to switch the internal powersupply voltage between Int.VccA and B, and dummy GNDA and B in such acase.

Referring to FIG. 68, the structure will be described. A p channeltransistor Tr101 receives at its source a reference signal X1 forgenerating an internal power supply voltage Int.VccA, at its gate aclock signal {overscore (φ)}, and has its drain connected to a comparinginput and of a comparing circuit 84. An output from comparing circuit 84is connected to the gate of a p channel transistor Tr103. The transistorTr103 receives at its source an external power supply voltage Ext.Vcc,and has its drain connected to a reference input end of comparingcircuit 84 and provides the internal power supply voltage Int.Vcc A. Areference signal X2 for generating an internal power supply voltageInt.VccB is applied to a comparing input end of a comparing circuit 85,and the output from comparing circuit 85 is connected to the gate of a pchannel transistor Tr104. The p channel transistor Tr104 receives at itssource the external power supply voltage Ext.Vcc, and has its drainconnected to a reference input end of comparing circuit 85 and providesan internal power supply voltage Int.VccB. Between the comparing inputends of comparing circuits 84 and 85, a p channel transistor Tr102 isconnected, to the gate of which a clock signal φ is applied.

An n channel transistor Tr110 receives at its source a reference signalY1 for generating the level of a dummy GNDA, at its gate the clocksignal φ, and the transistor has its drain connected to a comparinginput end of a comparing circuit 87. The output of comparing circuit 87is connected to the gate of n channel transistor Tr112, of which sourceis connected to an external GND. An n channel transistor Tr112 has itsdrain connected to a reference input end of comparing circuit 87 andprovides the dummy GNDA. A reference signal Y2 for generating the levelof a dummy GNDB is applied to a comparing input end of a comparingcircuit 86, and the output of comparing circuit 86 is connected to thegate of n channel transistor Tr111. The source of this transistor isconnected to the external GND. The n channel transistor Tr111 has itsdrain connected to the reference input end of comparing circuit 86 andprovides the dummy GNDB. Between comparing input ends of comparingcircuits 86 and 87, an n channel transistor Tr109 is connected, to thegate of which the clock signal {overscore (φ)} is applied.

Further, between the internal power supply voltage Int.VccB and thedummy GNDB, a series circuit of p channel transistor Tr105 and an nchannel transistor Tr107 as well as a series circuit of a p channeltransistor Tr106 and an n channel transistor Tr108 are connected. The pchannel transistor Tr105 and the n channel transistor Tr107 receive attheir gates an input signal, have their drains connected to the gates ofp channel transistor Tr106 and n channel transistor Tr108, respectively,and p channel transistor Tr106 and n channel transistor Tr108 have theirdrains used as the outputs.

FIG. 69 is a time chart showing the operation of the circuit shown inFIG. 68. In use, the signal φ attains to the “H” level as shown in (a)of FIG. 69, and {overscore (φ)} attains to the “L” level as shown in (b)of FIG. 69. Consequently, p channel transistor Tr101 of FIG. 68 isrendered conductive, the reference signal X1 is applied to comparingcircuit 84, and reference signal X2 is applied to comparing circuit 85.Comparing circuit 84 compares the internal power supply voltage Int.VccAwith the reference signal X1, the p channel transistor Tr103 lowers theexternal power supply voltage Ext.Vcc in accordance with the outputtherefrom, and provides an internal power supply voltage Int.VccA whichis lower than the external power supply voltage Ext.Vcc as shown in (c)of FIG. 69. At this time, comparing circuit 85 controls p channeltransistor Tr104 so that the external power supply voltage Ext.Vcc islowered, and hence the internal power supply voltage Int.VccB of whichlevel is lower than the internal power supply voltage Int.VccA isprovided.

Meanwhile, as the signal {overscore (φ)} attains to “L”, n channeltransistor Tr109 is rendered conductive, n channel transistor Tr110 isrendered non-conductive, and the reference signal Y2 is applied tocomparing circuits 86 and 87. Comparing circuit 87 compares thereference signal Y2 with the dummy GNDA, and provides the dummy GNDAwhich has higher potential than the external GND as shown in (d) of FIG.69. Meanwhile, comparing circuit 86 controls n channel transistor Tr111,so that the dummy GNDA having higher level than the dummy GNDA isprovided.

Meanwhile, p channel transistor Tr105 and n channel transistor Tr107operate at a potential V_(A) between the internal power supply voltageInt.VccB and the dummy GNDB, while p channel transistor Tr106 and nchannel transistor Tr108 operate at a potential V_(B) between theinternal power supply voltage Int.VccA and the dummy GNDA.

When not in use, if a clock signal as a chip operating signal is notprovided for a prescribed time period as shown in (a) of FIG. 70, thesignal φ falls to “L” level, the signal {overscore (φ)} rises to the “H”level, and when the device is set to the operative state and the clocksignal is input, then the signal φ rises to “H” level, and {overscore(φ)} falls to “L” level. As the signal φ falls to “L” level, p channeltransistor Tr102 of FIG. 68 is rendered conductive, the signal{overscore (φ)} attains to “H” level, p channel transistor Tr101 isrendered non-conductive, and reference signal X2 is applied to comparingcircuits 84 and 85. Therefore, comparing circuit 85 renders p channeltransistor Tr104 conductive, thus providing internal power supplyvoltage Int.VccB.

Meanwhile, n channel transistor Tr109 is rendered non-conductive and nchannel transistor Tr110 is rendered conductive, so that comparingcircuit 87 renders conductive n channel transistor Tr112, so as to raisethe potential of the ground GNDB. Consequently, an output bufferconsisting of p channel transistor Tr106 and n channel transistor Tr108operate at the potential Vb. More specifically, since it is preferred toincrease the speed of operation of the output buffer during use even ifthere is little leak current, the device is operated at a potential ofVb. When not in use, the device may be operated at a potential V_(A)which is lower than V_(B), suppressing leak current.

FIG. 71 shows a principle of an embodiment in which operations of thesubstrate bias voltage and the boosted power supply circuit arecontrolled when not in use. In the example shown in FIG. 71, the levelof the substrate bias voltage V_(BB) is made deeper than the externalGND and the level of the boosted voltage Vpp is made higher than theexternal power supply voltage Ext.Vcc during use, while the level of thesubstrate bias voltage Vbb is made shallower than the external GND andthe level of the boosted voltage Vpp is made lower than the externalpower supply voltage Ext.Vcc when not in use.

FIG. 72 shows an example of a circuit controlling the substrate biasvoltage. Referring to FIG. 72, a signal {overscore (φ)} is applied tothe gate of an n channel transistor Tr131. The drain of n channeltransistor Tr131 is connected to a comparing input end of a comparingcircuit 89. A constant current from a constant current source 88 issupplied to the drain of n channel transistor Tr131. Comparing circuit89 has its reference input end grounded. An output from comparingcircuit 89 is connected to a substrate potential generating circuit 90,and the output of substrate potential generating circuit 90 is connectedto the source of n channel transistor Tr131 and provides the substratebias voltage Vbb.

As already described with reference to FIG. 69, the signal {overscore(φ)} attains to “L” level during use, rendering less conductive nchannel transistor Tr131. Consequently, the resistance value between thedrain and the source of n channel transistor Tr131 increases, currentflow from the constant current source 88 is suppressed, and thereforethe potential at the comparing input end of comparing circuit 89 rises.Comparing circuit 89 compares a comparison input with the low level ofthe word lines, the output of which is applied to the referencepotential generating circuit 87. Accordingly, as shown in FIG. 71,reference potential generating circuit 87 makes higher the substratebias voltage V_(BB) than the external GND.

When not in use, the signal {overscore (φ)} attains to the “H” level andn channel transistor Tr131 is rendered conductive. Therefore, theresistance value decreases, allowing easier flow of the current from theconstant current source 88. As a result, the comparison input voltage ofcomparing circuit 89 decreases, and substrate potential generatingcircuit 90 makes shallower the substrate bias voltage V_(BB) than theexternal GND, in accordance with the output from comparing circuit 89.

FIG. 73 is a schematic diagram showing an example in which the potentialof the boosted voltage Vbb is controlled to be different when in use andnot in use. The p channel transistor Tr134 receives at its gate thesignal φ, and has its source connected to a comparing input end of acomparing circuit 92. A comparing input of comparing circuit 92 isconnected to the drain of an n channel transistor Tr133 of which sourceis grounded and of which gate is connected to the drain and the gate ofan n channel transistor Tr132. The n channel transistor Tr132 has itssource grounded, and n channel transistor Tr132 receives at its drain aconstant current from a constant current source 91. To a reference inputend of comparing circuit 92, the external power supply voltage Ext.Vccis applied, and the output from comparing circuit 92 is connected to aboosted voltage generating circuit 93. The output of boosted voltagegenerating circuit 93 is connected to the drain of a p channeltransistor Tr134, and the boosted voltage Vpp is provided.

The operation will be described. A current from constant current source91 flows to n channel transistor Tr132, and a current having the samevalue also flows to n channel transistor Tr133. Since the signal φattains to the “H” level during use, p channel transistor Tr134 isrendered less conductive, increasing the resistance value. As a result,the voltage at the comparing input of comparing circuit 92 attains toVpp-Ir (where r represents the resistance of p channel transistorTr134). Comparing circuit 92 compares the external power supply voltageExt.Vcc and the comparing input, and elevates the boosted voltage Vppgenerated from boosted voltage generating circuit 93.

When the signal φ attains to the “L” level as the device is not in use,p channel transistor Tr134 is rendered more conductive, lowering thecomparing input voltage of comparing circuit 92. Therefore, comparingcircuit 92 lowers the boosted voltage Vpp generated from boosted voltagegenerating circuit 93.

As described above, in the embodiment shown in FIGS. 72 and 73, theboosted voltage Vpp is raised and the substrate bias voltage Vpp is madedeeper than the external GND in use, while not in use, the boostedvoltage Vpp is made lower and the substrate bias voltage V_(BB) is madeshallower, so as to reduce leak current at the time of non-use.

FIG. 74 shows a circuit in an LSI to which the present invention inaccordance with the eighth aspect is applied. In the example shown inFIG. 74, based on a plurality of reference potentials generated from oneand same reference potential generating circuit 100, differentpotentials are supplied to various circuit groups in the LSI, so thatthe circuits operate at arbitrary potentials respectively. Morespecifically, the reference potential generating circuit 100 isstructured in the similar manner as shown in FIG. 62 in which resistorsR2, R3 and R4 are connected in series between the drain of p channeltransistor Tr85 and the ground, and from respective nodes, referencevoltages Vref1, Vref2, Vref3 and Vref4 are generated. In the LSI,circuit groups A to F are provided. Circuit group A operates at apotential between the external power supply voltage Ext.Vcc and theexternal GND, and circuit group B operates at a potential between theexternal GND and the internal power supply voltage Int.Vcc1 which is setbased on the reference voltage Vref1 set by a potential setting circuit111. Circuit group C operates at a potential between the external GNDand an internal power supply voltage Int.Vcc2 which is set based on thereference voltage Vref2 set by a potential setting circuit 112. Circuitgroup D operates at a potential between an internal power supply voltageInt.Vcc3 which is set based on the reference potential Vref1 by apotential setting circuit 113 and a dummy GND1 which is set based on thereference voltage Vref3 by a potential setting circuit 116. Similarly,circuit group E operates at a potential between the internal powersupply voltage INT.Vcc3 set based on the reference voltage Vref1 by apotential setting circuit 114 and the dummy GND1 set based on thereference voltage Vref3 by a potential setting circuit 117. Circuitgroup F operates at a potential between the internal power supplyvoltage Int.Vcc2 set based on the reference voltage Vref3 by a potentialsetting circuit 115 and the dummy GND set based on the reference voltageVref3 by a potential setting circuit 118.

FIG. 75 shows a first embodiment in accordance with a ninth aspect ofthe present invention. The embodiment shown in FIG. 75 is an improvementof the embodiment shown in FIG. 22. More specifically, in the embodimentshown in FIG. 22, when active, differential amplifying circuit 8 and nchannel transistor Tr3 are activated so as to keep the potential of thedummy GND line 30 at a constant potential, and in the standby state, itis clamped by n channel transistor Tr2 at the threshold voltage Vthnthereof. In that case, the reference voltage Vref of the differentialamplifying circuit 8 is Vref=Vthn. However, it is possible that in thestandby state, the level of the dummy GND line 30 lowers undesirably,because of the sub threshold current of n channel transistor Tr2.

In view of the foregoing, the embodiment shown in FIG. 75 is made toprevent lowering of the level of dummy GND line 30. For this purpose, ann channel transistor Tr121 is connected between the dummy GND line 30and the external power supply voltage Ext.Vcc. A level compensationcircuit of dummy GND line 30 is formed by n channel transistors Tr2 anTr121. The gate potential of n channel transistor Tr121 is set at 2Vref.Therefore, the sub threshold current flowing through n channeltransistor Tr2 is equivalent to the sub threshold current flowingthrough n channel transistor Tr121, and therefore the potential of dummyGND line 30 can be kept constant.

FIG. 76 is a schematic diagram of a circuit for generating the referencevoltage Vref and the gate voltage Vp shown in FIG. 75. Referring to FIG.76, a constant current source 121 and resistors R11 and R12 areconnected in series between the external power supply voltage Ext.Vccand the external GND, from the node between constant current source 121and resistor R11, the potential Vp=2Vref is provided, and from the nodebetween resistors R11 and R12, the potential Vref=Vthn is provided.

FIG. 77 shows an example in which resistors R11 and R12 shown in FIG. 76are provided by transistors. More specifically, resistor R11 is replacedby an n channel transistor Tr122, and resistor R12 is replaced by an nchannel transistor Tr123. In this example, a back gate bias potential isset such that the transistors Tr122 and Tr123 have the same thresholdvoltage.

FIG. 78 shows another embodiment in accordance with the ninth aspect ofthe present invention. In this embodiment shown in FIG. 78, in place ofn channel transistor Tr121 shown in FIG. 75, a p channel transistorTr125 is connected between the external power supply voltage Ext.Vcc andthe dummy GND line 30. In this case, the gate potential of p channeltransistor Tr125 is biased to Vcc-Vt. Other operations are the same asthose of FIG. 75.

FIG. 79 shows a still another embodiment in accordance with the ninthaspect of the present invention. In the embodiment shown in FIG. 79, inplace of n channel transistor Tr121 of FIG. 75, a p channel transistorTr126 is connected between the line of the external power supply voltageExt.Vcc and the dummy GND line 30. The operation is the same as that ofFIG. 75 except that the gate potential of p channel transistor Tr126 isbiased to Vcc-Vp.

FIG. 80 shows an improvement of the embodiment shown in FIG. 78. Morespecifically, a gate voltage Vp is applied to the gate and the drain ofp channel transistor Tr125. Since the sub threshold current is suppliedfrom voltage Vp, the voltage VP is supplied from the circuit shown inFIG. 77, and therefore it must have current compensating ability.

FIG. 81 shows an improvement of the example shown in FIG. 80. Morespecifically, the back gate n channel transistor Tr125 shown in FIG. 80is replaced by a p channel transistor Tr127 with a back gate.

In the embodiments in accordance with the ninth aspect of the presentinvention described above, Vp was set to Vp=2Vref and the sub thresholdcurrents of the transistors are set to the same value. However, thelevel of the dummy GND line 30 can be kept constant provided that thesub threshold current of n channel transistor Tr124 is equal to that ofn channel transistor Tr127 or to that of p channel transistor Tr125,even if Vp is not 2Vref. For this purpose, threshold values and sizes ofrespective transistors may be adjusted.

FIG. 82 is a schematic block diagram showing a first embodiment inaccordance with a tenth aspect of the present invention and FIG. 83 is atime chart showing the operation thereof.

As already described, by setting the line of low level potential ofinternal circuit 5 to the level Vss′ of the dummy GND, the disturbrefresh time (data retention time) of the memory cell MC can be madelonger. However, this means that the test time of the disturb refreshtime before shipment becomes longer, increasing the cost of testing.Therefore, the tenth aspect of the present invention is directed toreduction of test time while maintaining the high performance that thedisturb refresh time during normal operation is long.

Referring to FIG. 82, the dummy GND level Vss′ generated by dummy GNDlevel generating circuit 19 is applied to the dummy GND line 30. Thereis provided an n channel transistor Tr127 having its drain connected todummy GND line 30, its source grounded and receiving at its gate a testmode enable signal φtest indicating the entrance of test mode.

The operation will be described. After normal operation mode, at a WCBRtiming (Write and CAS Before RAS) at which the signals {overscore (WE)}and {overscore (CAS)} fall earlier than the signal {overscore (RAS)}, ahigh voltage level higher by several v than the power supply voltagelevel Vcc is input to a designated address pin. The WCBR and the inputof the high voltage level to the designated address pin set a disturbrefresh acceleration test mode.

When the set timing is confirmed and the set cycle starts, a test modeenable signal φtest is generated. When the signal φtest is input to thegate of n channel transistor Tr127, n channel transistor Tr127 turns onand pulls the dummy GND line 30 to the low level of the word lines Vss.At this time, the line of low level potential for sense amplifier is notat the dummy GND level Vss′ (0.5V) but at the true low level of the wordlines Vss (0V) supplied from an external pad. Therefore, the level ofthe bit line BL shown in FIG. 44 is also set not to the dummy GND levelVss′ (0.5V) but to the low level of the word lines Vss (0V), and thepotential Vgs of the gate (word line WL) with respect to the source (bitline BL) of the memory cell transistor MT changes from a negativevoltage of −0.5V to 0V. Therefore, sub leak current of memory celltransistor MT increases, degrading disturb refresh characteristic of thememory cell MC. Accordingly, when the operation enters this mode,disturb refresh is accelerated, reducing the test time.

Thereafter, when the CBR (CAS Before RAS) timing at which the signal{overscore (CAS)} falls earlier than {overscore (RAS)} is confirmed, thetest mode enable signal φtest falls, n channel Tr127 turns off inresponse, and the dummy GND line 30 is again set to the dummy GND levelVss′ (0.5V). Then the operation returns to the normal mode.

FIG. 84 is a schematic block diagram showing a second embodiment inaccordance with the tenth aspect of the present invention and FIG. 85 isa schematic diagram showing the structure of the dummy GND levelgenerating circuit 19 shown in FIG. 84. Basic structure is the same asthat of the first embodiment. However, it is different in that the testmode enable signal φtest is also input to the dummy GND level generatingcircuit 19. The reason for this is to stop charging circuit 19 a by thetest mode enable signal φtest when the acceleration test mode is enteredin such a dummy GND level generating circuit 19 that includes a chargingcircuit 19 a, so as to compensate for excessive lowering of the dummyGND line 30.

More specifically, dummy GND level generating circuit 19 includes acharging circuit 19 a and a discharging circuit 19 b. Charging circuit19 a includes a differential amplifying circuit 71, n channeltransistors Tr3 and Tr129, and a p channel transistor Tr128.Differential amplifying circuit 71 has its inverted input node connectedto dummy GND line 30, and its non-inverted input node connected toreceive the reference potential Vref (=Vss′). The n channel transistorTr3 has its gate connected to an output node of differential amplifyingcircuit 71, its drain connected to receive the power supply voltage Vcc,and its source connected to dummy GND line 30. The n channel transistorTr129 receives at its gate the test mode enable signal φtest, has itsdrain connected to the output node of differential amplifying circuit 71and its source grounded. The p channel transistor Tr128 receives at itsgate the test mode enable signal φtest, at its drain the power supplypotential Vcc, and has its source connected to the power supply node 71a of differential amplifying circuit 71. Discharging circuit 19 bincludes n channel transistors Tr1 and Tr2. The n channel transistor Tr1and Tr2 have their drains both connected to dummy GND line 30 and theirsources both grounded. The n channel transistor Tr1 has its gateconnected to dummy GND line 30 and n channel transistor Tr2 receives atits gate an internally generated signal φ.

When the test mode enable signal φtest is at the “L” level in the normalmode, p channel transistor Tr128 turns on, and n channel transistorTr129 turns off. When p channel transistor Tr128 turns on, the powersupply potential Vcc is applied to the power supply node 71 a ofdifferential amplifying circuit 71, and differential amplifying circuit71 is activated. When the potential of dummy GND line 30 becomes lowerthan the reference potential Vref, the output from differentialamplifying circuit 71 attains to the “H” level, turning n channeltransistor Tr3 on, whereby the dummy GND line 30 is charged. If thepotential of dummy GND line 30 becomes higher than the referencepotential Vref, the output from differential amplifying circuit 71attains to the “L” level, turning n channel transistor Tr3 off, wherebycharging of dummy GND line 30 is stopped.

When the test mode enable signal φtest is at the “H” level in theacceleration test mode, p channel transistor Tr128 turns off and nchannel transistor Tr129 turns on. When p channel transistor Tr128 turnsoff, application of the power supply potential Vcc to the power supplynode 71 a of differential amplifying circuit 71 is stopped, anddifferential amplifying circuit 71 is inactivated. Since n channeltransistor Tr129 turns on, the output node of differential amplifyingcircuit 71 is grounded, turning off n channel transistor Tr3. Thuscharging of dummy GND line 30 is stopped. The operation of thedischarging circuit 19 b is the same as in the dummy GND levelgenerating circuit shown in FIG. 2, and therefore description thereof isnot repeated.

The timings for setting and resetting the acceleration test mode are thesame as those of the first embodiment, and similar effects as the firstembodiment can be obtained.

FIG. 86 is a schematic block diagram showing a third embodiment inaccordance with the tenth aspect of the present invention. In thisembodiment, different from the first and second embodiments, the nchannel transistor Tr127 receiving at its gate the test mode enablesignal φtest is not provided separately, but an n channel transistor Trof a large size already provided in dummy GND level generating circuit19 for pulling the dummy GND line 30 to the low level of the word linesVss is used. Tough the internally generated signal φ has been input tothe gate of n channel transistor Tr2 in the above described embodiments,the internally generated signal φ and the test mode enable signal φtestare input to the NOR gate 130, and the output of the NOR gate 130 isinput to the gate of n channel transistor Tr2 in this embodiment.

The timings for setting and resetting the accelerated test mode are thesame as those shown in the first embodiment. In normal operation, sincethe test mode enable signal φtest is at the “L” level, n channeltransistor Tr2 is controlled by the internally generated signal φ.However, when the operation enters the acceleration mode set cycle andthe test mode enable signal φtest attains to the “H” level, the gate ofn channel transistor Tr2 attains to the “H” level regardless of thestate of the internally generated signal φ. Therefore, n channeltransistor Tr2 turns on, pulling the dummy GND line 30 to the low levelof the word lines Vss.

The similar effects as in the first embodiment can be obtained by thisembodiment also.

FIG. 87 is a schematic diagram showing a fourth embodiment in accordancewith the tenth aspect of the present invention, and FIG. 88 is a timechart showing the operation thereof. Basic circuit configuration of thisembodiment is the same as that in the first, second and thirdembodiments above, and though not shown in FIG. 87, dummy GND levelgenerating circuit 19 and n channel transistor Tr127 are provided aswell. In this embodiment, in addition to these components, an n channeltransistor Tr130 is provided, which receives at its drain the powersupply potential Vcc, has its source connected to the dummy GND line 30and receives at its gate a one shot pulse signal φtest′, which will bedescribed later. The n channel transistor Tr130 assists the operationfor setting dummy GND line 30 which has been pulled down to the lowlevel of the word lines Vss (0V) in the accelerated test mode back tothe normal dummy GND level Vss′ (0.5V) in the reset cycle.

The operation will be described. In the accelerated test mode, test modeenable signal φtest is at the “H” level, n channel transistor Tr127 ison and dummy GND line 30 is pulled down to the low level of the wordlines Vss (0V). Then, when the operation enters the reset cycle at thetiming shown in the first embodiment, the test mode enable signal φtestfalls to the “L” level, and n channel transistor Tr127 turns off. Basedon the fall of the test mode enable signal φtest, the one shot signalφtest′ is generated, which is input to the gate of n channel transistorTr130. In response, n channel transistor Tr130 is rendered conductive,and dummy GND line 30 is quickly pulled up to the dummy GND level Vss′(0.5V).

Though an n channel transistor is used as the charging transistor Tr130,a p channel transistor may be used. However, in that case, it isnecessary to invert the one shot pulse signal φtest′.

FIG. 89 is a schematic block diagram showing a fifth embodiment inaccordance with the tenth aspect of the present invention. Referring toFIG. 89, in this embodiment, a switch 132 is provided which switches inaccordance with the test mode enable signal φtest. One switch terminal132 of switch 132 is connected to an output of dummy GND levelgenerating circuit 19, the other switch terminal 132 b is connected to agrounded external pad 131, and a common terminal 132 c is connected tothe dummy GND line 30.

When the test mode enable signal φtest is at the “L” level in the normalmode, the common terminal 132 c of switch 132 is connected to one switchterminal 132 a, and dummy GND line 30 is set to dummy GND level Vss′.When the test mode enable signal φtest attains to the “H” level in theaccelerated test mode, the common terminal 132 c of switch 132 isconnected to the other switch terminal 132 b, and dummy GND line 30 isset to the low level of the word lines Vss.

The switch 132 consists of two n channel transistors Tr131 and Tr132 asshown in FIG. 90, for example. The drains of n channel transistors Tr131and Tr132 serve as switch terminals 132 b and 132 a, respectively, thesources both serve as the common terminal 132 c, and the gates receivethe test mode enable signal φtest and complementary signal {overscore(φt)}, respectively. In this embodiment also, the similar effects as inthe first embodiment can be obtained. This embodiment may be combinedwith the fourth embodiment.

FIG. 91 is a block diagram showing a DRAM chip configuration inaccordance with a sixth embodiment in accordance with the tenth aspectof the present invention and FIG. 92 shows, in enlargement, the mainportion thereof. Referring to FIGS. 91 and 92, the DRAM chip includes aplurality of memory array areas 141 and a peripheral circuit area 142provided therebetween. Each memory array area 141 includes a pluralityof sub arrays 143 arranged in the row direction, a plurality of senseamplifier bands 144 provided between and on both sides of, the subarrays 143, a row decoder 145 and a column decoder 146.

Sub array 143 includes a plurality of memory cells (not shown) arrangedin rows and columns, word lines (not shown) provided for respectiverows, and bit line pairs BL, {overscore (BL)} provided for respectivecolumns. Sense amplifier band 144 includes a sense amplifier 2 providedcorresponding to each column, and each sense amplifier 2 is connected tothe corresponding bit line pair BL, {overscore (BL)}. The senseamplifiers 2 of each sense amplifier band 144 are commonly connected tothe sense drive line SN.

Peripheral circuit area 142 includes dummy GND line 30 to which dummyGND level Vss′ is applied by dummy GND level generating circuit 19, aground line 147 which is grounded through external pad 131, and switches132 provided respectively for the sense amplifier bands 144. Switch 132includes an n channel transistor Tr132 connected between dummy GND line30 and sense drive line SN, and an n channel transistor Tr131 connectedbetween the ground line 147 and sense drive line SN. The n channeltransistors Tr132 and Tr131 are controlled by sense amplifier activatingsignals S0N1 and S0N2, respectively.

Sense amplifier activating signal S0N1 is provided from a gate circuit161 which receives sense amplifier activating signal S0N and test modeenable signal φtest. Gate circuit 161 provides sense amplifieractivating signal S0N as it is when the test mode enable signal φtest isat the “L” level in the normal mode. When test mode enable signal φtestis at the “H” level in the test mode, gate circuit 161 always provides“L” level regardless of the sense amplifier activating signal S0N.

Referring to FIG. 94, sense amplifier activating signal S0N2 is providedfrom an AND gate circuit 162 which receives sense amplifier activatingsignal S0N and test mode enable signal φtest. When the test mode enablesignal φtest is at the “L” level in the normal mode, AND gate circuit162 always provides the “L” level regardless of the sense amplifieractivating signal S0N. When the test mode enable signal φtest is at the“H” level in the test mode, AND gate circuit 162 provides senseamplifier activating signal S0N as it is. The operation will bedescribed. In the normal mode, n channel transistor Tr132 turns on inresponse to the rise of sense amplifier activating signal S0N1 to the“H”, and dummy GND level Vss′ is applied to sense drive line SN. In thetest mode, n channel transistor Tr131 turns on in response to the riseof the activating signal S0N2 to the “H” level, and sense drive line SNis grounded.

Similar effects as in the first embodiment can be obtained in thisembodiment.

If dummy GND line 30 and ground line 147 are formed as mesh respectivelyto cover the memory area 141 and the sense drive lines SN of the senseamplifiers and the dummy GND line 30 and the ground line 147 areconnected thereto by means of a plurality of switches 132,interconnection resistances of interconnections SN30 and 147 can bereduced, preventing floating of the potential derived frominterconnection resistance.

Here, sense amplifiers 2, switches 132 and the like are formed on thesurface of a silicon substrate, and ground lines 147 and dummy GND lines30 are insulated from each other and successively stacked above thesilicon substrate.

FIG. 96 is a partially enlarged schematic diagram showing a DRAM chipconfiguration in accordance with a seventh embodiment in accordance withthe tenth aspect of the present invention. Referring to FIG. 96, in theDRAM chip, the peripheral circuit area 142 includes n channeltransistors Tr provided respectively for the sense amplifier bands 144,a dummy GND line 30 to which dummy GND level Vss′ is applied from dummyGND level generating circuit 19, and an n channel transistor Tr127 forgrounding the dummy GND line 30 in response to the test mode enablesignal φtest. Each n channel transistor Tr133 is connected between sensedrive line SN of each sense amplifier band 144 and dummy GND line 30,and receives at its gate the sense amplifier activating signal S0N. Then channel transistor Tr127 is connected between external pad 131 anddummy GND line 30, and receives at its gate the test mode enable signalφtest.

The operation will be described. In the normal mode, test mode enablesignal φtest is at the “L” level, n channel transistor Tr127 is off anddummy GND line 30 is set to dummy GND level Vss′. In the test mode, testmode enable signal φtest attains to the “H” level, n channel transistorTr127 turns on and dummy GND line 30 is grounded. When sense amplifieractivating signal SON attains to the “H” level, n channel transistorTr133 turns on and sense amplifier 2 is activated.

In this embodiment also, the same effects as in the first embodiment canbe obtained. As compared with the sixth aspect, the number oftransistors and the number of interconnections can be reduced.

Referring to FIG. 97, if the dummy GND line 30 is formed as a mesh tocover memory cell area 141, and sense drive lines SN of respective senseamplifier bands 144 and the dummy GND line 30 are connected by means ofa plurality of switches 132, interconnection resistance ofinterconnections SN and 30 can be reduced, and floating of potentialderived from interconnection resistance can be prevented.

FIG. 98 is a block diagram showing an eighth embodiment in accordancewith the tenth aspect of the present invention, and FIG. 99 is a timechart showing the operation thereof. Referring to FIG. 98, thisembodiment includes a memory array 150, a word driver 10 for drivingword lines WL of the memory array 150, and a switch 132. To a high levelpotential line 10 a of word driver 10, a high supply potential Vpp whichis boosted from power supply potential Vcc is applied. A low levelpotential line 10 b of word driver 10 is connected to a common terminal132 c of switch 132, one switching terminal 132 a of switch 132 isconnected to the ground line 147 and the other switch terminal 132 b ofswitch 132 is connected to dummy GND line 30. Switch 132 is the same asthat shown in FIG. 89, and it is connected by test mode enable signalφtest.

The operation will be described. From the normal mode, the operationenters the accelerated test mode set cycle at the timing shown in FIG.83, and when test mode enable signal φtest rises to the “H” level, thelow level potential line 10 b of word driver 10 is switched from the lowlevel of the word lines Vss (0V) to the dummy GND level Vss′ (0.5V) bymeans of switch 132. Consequently, the non-selected level of word lineWL is raised from the low level of the word lines Vss (0V) to the dummyGND level Vss′ (0.5V), degrading the disturb refresh characteristic.This reduces the time necessary for the refresh test. Then, when theoperation enters the reset cycle, the test mode enable signal φtestfalls to the “L”, the low level potential line 10 b of word driver 10 isswitched again to the low level of the word lines Vss (0V), and normaloperation resumes.

When combined with the first to seventh embodiments, the disturb refreshcharacteristic can be further degraded, further increasing the effect ofreducing necessary test time.

FIG. 100 is a block diagram showing a ninth embodiment in accordancewith the tenth aspect of the present invention. Referring to FIG. 100,this embodiment includes a memory array 150 formed in a well 151, a worddriver 10 for driving word lines WL of memory array 150, and a senseamplifier band 144 connected to bit line pairs BL, {overscore (BL)} ofthe memory array 150. This embodiment further includes a negativepotential generating circuit 152 for generating a negative potentialVbb, a grounded external pad 131, and a switch 132 for switching andsupplying the negative potential Vpp or the low level of the word linesVss to well 151. One switch terminal 132 a of switch 132 is connected toan output of negative potential generating circuit 152, the other switchterminal 132 b is connected to external pad 131, and the common terminal132 c is connected to well 151. Switch 132 is the same as that shown inFIG. 89, and it is controlled by test mode enable signal φtest.

The operation will be described. In the normal mode, test mode enablesignal φtest is at the “L” level, the common terminal 132 c of switch132 is connected to one switch terminal 132 a, and the negativepotential Vbb is applied to well 151 by negative potential generatingcircuit 151. Consequently, the sub leak current of memory celltransistor MT can be suppressed low, and disturb refresh characteristicof the memory cell MC can be maintained satisfactorily.

When accelerated test mode set timing starts at the timing shown in FIG.83, the test mode enable signal φtest rises to the “H” level, the commonterminal 132 c of switch 132 is connected to the other switch terminal132 b, and well 151 is grounded through external pad 131. Therefore, subleak current of memory cell transistor MT is increased, degradingdisturb refresh characteristic of memory cell MC. Thus the test time canbe reduced. Thereafter, when the operation enters the reset cycle, testmode enable signal φtest falls to the “L” level, and the negativepotential Vpp is again applied to well 151.

If combined with the first to eighth embodiments above, the disturbrefresh characteristic of the memory cell MC can be further degraded,and the effect of reducing the test time can be further improved.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. A semiconductor memory device comprising: a semiconductor substratehaving a first conductivity type and supplied with a ground potential; afirst well formed on said semiconductor substrate, having the firstconductivity type and electrically connected to said semiconductorsubstrate; a second well formed on said semiconductor substrate, havingthe first conductivity type and electricaly connected to saidsemiconductor substrate; a boosted low level potential applying circuitfor applying a boosted low level potential higher than a low levelpotential of a word line; a memory array including a memory transistorhaving a second conductivity type and formed on said first well, asource/drain of the memory transistor being connected to a bit line towhich the boosted low level potential is applied, and a gate of thememory transistor being coupled to the word line; and a peripheralcircuit including a peripheral transistor having the second conductivitytype and formed on said second well, a source of the peripheraltransistor being coupled to the ground, potential.